1988-1998 Cougars are supposed to have good IRS that works well in custom applications, it is also said to handle hi power levels. (Some guy had one behind a roots blown 392 Hemi.) I second the idea of useing 5.0 HO EFI.
I believe it's 89+ newer. TBirds are identical. If I chose to fool with one I'd look out for a 91 (or so) SuperCoupe. AKA a supercharged TBird. They had 4 wheel disc brakes and often a limited slip rear. I tossed around the idea for a Mustang swap, but decided it would way more work than I wanted to get into. The whole rear assembly just about drops out in one piece. The assembly looks way too wide for a Spit. Some serious measuring and modifying may prove me wrong, but I don't believe it would work out.
I'm trying not to bore you guys too much with my Project Warm Spit details (this is a Mustang forum after all), but damn you guys are a wealth of great info. I didn't know those cougs were IRS. That would be great, then Warm Spit would be Ford from end to end! I know that pretty much whichever IRS I go with will have to be chopped down for the spit width. That plus some serious offset in my rear rims!
I'm leaning toward a carburated 5.0 as opposed to EFI. I think it'll give me better flexibility in components and mounting, plus I'm ascared of EFI. We'll see, nothing is decided yet. For all I know, I'll end up restoring this thing bone stock! Hmmm ... 1300cc 48 hp, maybe not!/forums/images/icons/smile.gif
I flirted with the idea of independant rear suspension on my mustang and came to the conclusion that it did not have the bang for the buck for me. Your case is even stranger because your putting it in that little whatever it is. (BTW, FILL OUT YOUR BIO). Narrowing an IRS does not seem practicle to me. You will be changing the suspension geometry, cutting and welding a-arms. The thought of it frightens me. Why not just narrow a 9" and declare the rear end indestrctible. BTW, i've seen on the web a place that has brackets to mount lincoln rear disc brakes to a ford 9" so you wouldn't have to use the cadilac calipers.
I'm changing the 289 in my mustang out for a 5.0 High Output. I've gotten so acustomed to modern cars. Jump in it turn the key, it starts first time, hit reverse and gently go backwards... VS. the 289, ge in it and hit the gas 2 times, crank it a second till it fires, wait a few seconds and hit it off high idle down to the choke idle, hit reverse, stall... The new stuff for me just works easier. Plus for uniqueness there are not many projects like yours that has EFI. Not that you should need more power but the EFI 5.0 is very easy to add a supercharger and nitrous.
Not my project...but he lives near by and you should see what he is brewing...funky. IIRC, the hoist is what is suspending the rear section from the top and the sawhorses are actually just loading up the IRCfor measurements. If you look close to the second picture, you can see the "table" he built for this stage of his project.
LOOK at the madness...
and from the top...
The rear end is out of a Supra...he also front clipped the supra to his '65 and created a frame from front to back.