1990 4-banger to 351 or 5.0

[fiveohpatrol]

Don't sell the 5.0 that you have. Personally, I would build up the 302 and put it in there. The 351 does have more potential to make power, but the 302 can make plenty of power, and you have more aftermarket parts to choose from for it.

If your 5.0 came out of a mustang, then you won't need a whole lot of other parts to put it in your car (unlike doing the 351). You'll need the full exhaust system, motor mounts, all hoses and wiring, a bigger radiator, not sure about the driveshaft but that 7.5" rear will be toast within a week or so of some real power.
The 8.8" rear is a direct swap and can be found easily in junkyards.

I've probably forgotten many things, but I'm sure someone else will come by and help.

good luck with it