what are you looking to gain by replacing the engine..... if your looking for alot more power, I wouldn't recommend it unless you upgrade/modify the rest of the truck... If looking to go slightly larger, epic 18, cen 3.0 or O.S. 18 are pretty much drop in... I have a feeling the O.S. may be too much for the truck...
My main question is why... Are you looking for more speed or more torque? It makes a differrence as to what engine to get....