If the Jag V12 isn't in perfect shape, it'll cost you a bit to renovate it. But yeah, V12s are the best.
Cad or Olds' sitting on top of that transaxle aren't noted for being great handlers as the engine sits pretty high on the transaxle--raising the cars CG, which fouls things up like ride and handling.
Youu never hear about Fiero or Corvair guys raving about their cars handling after swapping in a Toronado drivetrain.
If they do, they're talking about how it drives to the grocery store.....
I'd stick the Caddy in front of a beefy 4 speed, and put it up front, but as far back as possible. Might want to look in the wrecking yard or craigslist and find a 93+ Thunderbird or Cougar front and rear suspension. The rear is independent, and you can get 4 wheel discs.
The 89 and up cars (MN12 chassis) have the independant rear, but things like wheel bolt circle and ring and pinion size come into play.