I got a 351 with headers into an olds cutlass. I pretty sure you can get it in a original 5.0 car.

 

 

the range of displacments that will bolt into a fox car with out having to resort to a gas axe are 140 to 700 cubic inches, In a V8 or I4 car you can go with a 2.3 (140 cid) all the way up to a max stroked 460 (700 cid), these are all bolt in. A 351 is no sweat by any measure. Just need to get a Fox body 351 oil pan and the appropriate headers (or using something like the TFS high port headers on a standard port cylinder head). The limiting factor with the BB are cylinder heads, I dont think you can comfortably fit a Boss 429 in there (although even this might be possible because the DOHC 4.6 is a darn wide engine and they fit in there)

If anything, you will probably have to use a cowl hood with any sort of decent intake, otherwise a 351 fits in there like it was made for it (95 R model anyone)

 

Heck, I gotta 351w in my '66, and it's a LOT tighter fitting than my '84!!!