5.9 Mag stock LB or rebuilt SB

i'd probably start with the rebuilt short block, you can easily get to 400hp on stock compression & head with just a cam swap, intake and headers. aftermarket heads would make it a walk in the park.
if you go for the long block, magnums are easy on parts (except cam bearings evidently) and you should be able to get away with a soft parts fluff and fold. stab in a new cam and timing chain and you're set on the short block, just add your go fast goodies and you'll be ready to party.