The company that made the kits was called Ultimate Customs based in Ajax, Ontario. I have heard that the old owner Loyd still has all of the templates, jigs, etc and still produces them in limited qantities. I do know of one Ultimate Customs 5.0 swap kit that is brand new in the box. It is not cheap, around $300 (I think, that's what the guys wants). I used this kit and ran problem free for over 8 years. Today, acording to the new owner, the truck runs 12's with over 500 hp still with the Ultimate Customs kit.
You do not need to do any modifications, NO MODIFICATIONS to the engine bay to do the 5.0 swap. The factory rear end is fine, just do not run blocks or mono leafs. For the wiring harness, use a Painless Harness and save yourself time and a s%#t load of frusteration.