Re: V12 or V8
The Jaguar V12 is most likely too long to fit under the hood of a Jeep. The Viper engine is actually a V10, not a V12 and it's a tight squeeze. The Jag engines are very long. It is true that a Jaguar V12 will bolt up to a Chevy TH400. The shop I worked for in college replaced the V12 in a Jag XJ12 with a small block Chevy engine as it was much cheaper than rebuilding the V12. There was a lot more room in front of the engine after the swap. Unfortunately the TH400's in Jeeps don't have the same bolt pattern as a Chevy TH400 and the V12 will not bolt up to them.
'75 CJ5,258 w/Howell EFI,T18a,4.27's,33's,On-board air,Warn 8274