If you gotta rebuild both, the engine is a wash. both can be stroked, thr 340 to 416 and the 383 to 431 or 496. The big block route will require mounts, new trans, etc. The 340 obviously will be cheaper due to less add ons. I have a 383 in my duster. It runs good but i'm thinking about a stroker for it. If it was mine..... I'd do the stroked 340. 500 hp could be gotten easily on pump gas. How much more do you need?
don