Why go to the work of converting to a Chevy 350 when the factory made them with an AMC 304 - which allows a 260, 390, or 401 to be dropped right in. A 401 HO will give a 350 Chevy of the period a REAL run for it's money.
I owned a '75 Pacer. Rode like a limo, handled pretty good too - but the front "K" frame made a good road grader. We rallyed it a few times early in the season while wewere still working the bugs out of the R12.
Just a Hornet with the end sawed off. With the 304 it was pretty hot - and there was a dealer in Michigan selling them withthe 401 HO (AMX) engine.
The early Javelin with the 343 was pretty quick. The later ones got a bit heavier, but with the 401 AMX GoPack they were scary.
Very under-appreciated "muscle" cars.