Hi Wayne, my numbers are fairly close to Peter’s. The Mark V service manual page A.6 shows the 2 1.2 at 18.2 mph per 1000 rpm for 4.55 rear end and the 3 1/2 at 19.35 for, I suspect, the 4.3 ENV rear end ratio specified on A.5. I have the 4.27 rear ratio on my current Mark V. Basically, it takes about 6 % more rpms for the 2 1/2 to run the same speed as the 3 1/2 if wheels and tires and tranny gears are all the same. I am cautious about running my car above 3000 rpm for more than a few minutes.
On a basic engine power needed to go a given speed, the 2 1/2 engine rated at 102 hp can be expected to run at 60 mph without trouble. My 40 hp Ford Model A will get in that range but my pucker factor keeps it with 45 mph limiter. The main issue will be the leisurely time to get to speed and the safety at that speed. You can tell I drive these cars with the fear of God in mind given safety elements of the car and also the surrounding traffic.
For tires I use the Excelsior Stahl Radials 6.5 16 tires available from Coker. I used to run the lovely Dunlop Roadspeed tires for many years, no longer available. I selected the Excelsior Stahl Radial after a lot of calculations to match the tire patch with the Dunlops so as to have steering strength not increased. I also chose the Excelsior Stahl Radial over many other choices because they are DOT rated for public roadway use and many classic car tires are not DOT rated.