The guys are all on the right track. All are do-able swaps a donor truck helps. 2wd 460's or older car motor will fit with simple junkyard parts. I have a 400hp one in a 78 supercab dually conversion and a 450hp one in my 250 with 39" tires both preform flawlessly and have alot of miles since install. My 79 Bronco runs a 415hp 400ci that was built using plans almost identical to the previos metioned ones. The motor some people forgot is the 390. Good power decent fuel milage and fairly reliable if maintained. Good luck. If you do decide to hop up the 6 banger regear the axles to help with tire size in the rough stuff.: I can find and ship all neccasry parts from Oregon yards for really resanable prices except motor. Can get several of each size for cheap but big $$$ to truck.