make your own rail kit and engine plates, then all you have to worry about is the exhaust
The Exhausts come up on Eprey and Cl a lot. Usually the older Hooker boat headers tho. The motor plates (4 point mount) are a little harder to come by (which I why I tuned GC onto the set he got as soon as I saw them). 3 point mounts are easier in some cases because they usually use the same side block mounts as a Mopar B or E body car but mate to normal 3 point stringer mounts. Then you just have to find the Mopar RB 3 point jet bellhousing. Better to look for the 4 points, to be honest.
Check with Jim at Cyclone Marine (818)890-1867, Or
Tom at Jet Boat Performance (805)466-4719, Or
Fatboys at (480)382-6311 (not your normal crew, they're local here, Mesa, and kinda nuts, but they have a LOT and you gotta work 'em down).
If no luck on the used 4 points, I THINK Tom at JPB has plates and rail kits available for the 361-440. At least I know he put one in another boat.

Other things you are gonna need are the logs or headers, obviously, and the PTO adaptor. Those are available new, as a last resort though.
Ray