go 6.0 def bigger after market..
the saying is true though no replacement for displacement..but seeing you want junk yard pulls and efi and them are your only 2 picks def go 6.0..now you think about a duramax? goobs of tq and would run threw any off road shit you would want untill you burried your axles..
After my stang is done im moving onto doing a mud/trail style truck..2.5 rockwells 50" tractor tires and a old 80s step side if i can find a clean one..It will def have a 468 bbc in it though with some juice.probley have iron oval port heads..make around 700-750 on pump gas with out the jug.
pretty much a twin to this truck
http://www.youtube.com/user/z71on46s#p/f/28/TLNp5DvxD44