scrimbo, do you have a link to that?
That’s just stupid. I run only Pepsi. RC in a pinch.
And a Moon Pie as leak stop
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2.5L Universal Aluminum Coolant Radiator Overflow Expansion Tank Reservoir Recovery Water Tank Bottle
Price went from $29 to $35, and size went from 2L to 2.5L. 
As usual, several good thoughts and ideas - thanks all!
I am not willing to give up any coolant capacity, and near as I can measure the stock tank is about 2 quarts.
Just be squinting and measuring the closest so far is SUM-300118 .(Summit Racing SUM-300118 Summit Racing™ Coolant Expansion Tanks | Summit Racing). It is 4" square and 8" tall.
The main things that interfere in that area are the air cleaner snorkel (won’t move) and the rat’s nest of charcoal canister lines (will move.) There are several vendors that sell this same size tank.
This is all part of a fairly major AC and radiator upgrade project that was supposed to happen at Christmas but was waylaid by the vac booster failure.
Once I get things taken apart (I am stalling because the weather is nice and the car is working great) I will have a better idea of what might work. Fortunately I have a friend with mad tig skillz so worst case we will fab something workable.
Boy I wish the weather was nice here in Pa. Light snow today. Car still hibernating.
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Converting quarts to liters: 2 quarts equal less two liters (1.89 liters), thus a 2 liter expansion tank would do it. If in doubt, go with the 2.5 liter aluminum expansion tank.
Years ago, probably 10 or so, I noticed 3 pin holes in my expansion tank, I removed the tank and brazed the pin holes up. No additional pins hole have developed, at least not that are visible with the tank installed.
I plan on removing the tank in the next month or two, when doing other work in the engine compartment, and will try to remember to fill the tank with water, then measure how much water the existing expansion tank holds.
I do know that, at least in my expansion tank, the coolant level is about 1-1/4" below the top of the tank (not below the tank’s cap rim, but about 1-1/4" below where the neck and tank meet, just measured it) when the engine is dead cold (i.e., not having been run overnight or longer), and that after having been run (until it reaches stabilized engine temperature) and cooled down enough to be able to remove the cap without any pressure in the tank, the coolant level is (from memory) only maybe halfway up the next of the tank.
Which brings up a question: I also just measured (with engine dead cold) the coolant in the crossbar tube, at the fill neck, and there is 1/2" depth of coolant in the crossbar tube. In an engine (5.3L V12) fully filled with coolant, and with the engine dead cold, how much coolant should be in the crossbar tube? Does that 1/2" depth of coolant in the crossbar tube indicate that the coolant is not fully filled?
What is the depth of coolant in the crossbar tube in your engine with a cold engine? Curious what is ‘typical’ depth of coolant in there, and what does it indicate?
If there are still no pin holes (in places not visible when the tank is installed), I will repaint the tank and put it back in. If there are any pin holes, that would be a good time to replace the tank with one of the aluminum tanks … which means measuring the existing tank and getting one with the same, or maybe slightly larger, capacity.
If you have a properly working atmospheric catch tank (as I do) the expansion tank should be full to the top, all the way to the cap, hot or cold. If you don’t (I think the early cars didn’t have them…?) then I am not sure. And I would install one. 
Regarding the crossbar, A bit of air, like a 1/4" or so below the “shelf” where the pressure cap seats is normal on my car. This is the highest point in the system so I don’t worry about it. And it could be a Schrodinger’s Cat thing, opening the cap could cause a bit of “outflow” via the air extraction tube thus creating the air gap. In operation, once up to temp, the expanding coolant would close that air gap pretty quickly and the air is pulled out via the extraction tube.
(And yes, before anyone pipes up, the pressure cap on the crossover tube is not acting as a pressure cap, I am just using those terms to describe the coolant levels.)
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I have spent quite a lot of time on the cooling system, which I found rather complicated when I bought my car. The photo will show a flow indicator that I bought to test different parts and hose runs on the car, so I hope you believe the following-
The main purpose of the metal tank secured to the LHS fender is to give the water pump coolant which is pumped through the engine and exits to provide the air purge tubing across the top of the radiator with fluid. That fluid, mixed with any air bubbles goes directly back into the metal tank, IOW, recirculates. The metal tank should be full at all times, including the neck, up to the cap seal.
The secondary function of the metal tank is to allow heated expanded coolant, possibly including air bubbles, a path , through the radiator pressure cap, to go to the plastic tank, hidden from view. As the engine goes through the hot/cool cycles the coolant is drawn back into the metal tank as the engine cools, which should help explain why the metal tank should always be full. All this presumes that the tubing connecting the various parts is in good shape with no leaks.
If the air purge system is working properly, the coolant level in the bypass tube should be level with the small tube sticking out, which leads to the banjo bolt above the radiator. Hot or cold, makes no difference.
I have made some changes to the air purge plumbing, which eliminates any air in my coolant completely. Too much to relate now!
As I type this, Bob’s post appeared. I pretty much agree with everything he said, FWIW!
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I’ve only recently begun doing my own work, and I am continuously learning new things to check for and look.
When I’ve got many things off to provide access to clean the engine off better than I did recently, I’ll be removing things to clean and repaint, at which time I can do, and check for, other things. I will also make sure that the coolant system is filled and bled properly.
Love it! Glad you are taking the plunge.
One of the best things I did for cooling system is to install a flushing T in the heater hose on the right rear of the engine, per the Book. You can just see it peaking out here:
Every time I service the cooling system a stupid amount of air burbles out of this T. (I crack the cap so it is just screwed on and can sit there and wobble.) I can’t imagine expecting all of this air to make it’s way out through some other orifice.
I also raise the front of the car about 6 inches.
There are a lot of smart people lurking around here, don’t hesitate to ask questions, I learn new stuff here every day.
Bob
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Bob, I see the flange with the chassis number on it is red, matching the car color. Is that its original color?
My car is also red, Sebring Red, but that flange is black. It looks like it quite possibly was painted over the red body color at some point in its history, if it was not originally done at the factory.
Yes original color. It was all original when I got it.
sorry I let it go… just looked up stainless coolant overflow tank… or a combination of words to that effect