Sonny Im gonna argue that the 460 is a good engine with some TLC and proper parts can be made into a great performer..replace the Carter carb with a Holley 650 Truck avenger, great carb right out of the box,doesnt have the bowl pivot problems either and is made for off road use and doesnt flood out..works for me,but Im not you..you do what works for you..u might also have some timing issues that are holding it back and that would also make it hard to start as well..I saw first hand an old junkyard 460 pulled from a stationwagon win 6 races in a lifted 1978 F350 after a timing chain set, race camshaft and a intake and carb swop.
I think the 351 would be a good replacement choice but you have the bellhousing to change out then too as the 351 has the small block pattern and the 460 you currently have has the big block..
BTW those older 1990s trucks are getting hard to find in good shape, just to FIND a 4x4 F350 dualie is a gold nugget if its not trashed or wrecked