My Series II XJ V-12 has an exhaust leak, I am hoping it is at the manifold / downpipe junction. I have the car up on the lift and am baffled as to how the nuts for the flange can be removed. The big ol’ coffee can cats are right in the way. I have searched the forums and all I can find is it is a real PIA. Yeah buddy! I purchased new downpipes from SNG Barratt to eliminate the cats (I’m in Arizona so no smog worries) and intend on building a new stainless steel system from the downpipes out the back. Summit Racing sells a builder kit with 4 - 45’s, 4 - 90’s and 4 - 4’ straight sections for about $500, plus individual bends can be purchased separately. I am thinking a pair of the smaller Flowmaster “turbo” style mufflers will give me the tone I am seeking. I would love to hear from anyone with experience doing this.
It’s a tricky and not pleasurable task 
On my XJS and Series III V12 some of the nuts can be undone from above, actually. Remove the air cleaners first and you’ll see.
Working from below you’ll need some extensions and flex joints and then, if you look carefully, you can ‘just’ see a couple pathways to get your socket on the nuts.
As I recall the left side is worse.
In my experience you’ll want an H-pipe or X-pipe between the two sides. Without, the exhaust tone is quite odd, in my opinion
I have 4 glass packs on my Series II V12. Two at the extreme rear and two under the rear seat area. I am pleased with the tone. Not loud, no drone.
Cheers
DD
If you go the glass packs some of them have louvers in the inner tubing, depending which way you place them in the system can make a difference to the sound emitted, open louvers to the oncoming gas’s flow will be quieter than t’other way round.
I have a Borla stainless system on my '74, no catalytics, but I agree you need long extensions and swivel sockets to get the downpipe nuts.
I later discovered the heat shields are really important and were not on the Borla system so I had to build my own.
I have a Borla stainless system on my '74, no catalytics,
Bob,
Do you happen to remember the Borla part number? Their website does not have an application for my XJ12.
No, I haven’t found the receipt but it was a long time ago. I think the kit came from Special Interest Car Parts but they are long gone.