budget street motor 360 or 383 for 71 duster

As others have stated...........go small block; you've already got brackets, mounts, tranny, so no money spent there.

Power level you are looking for is an easy target to hit. Your short block, more cam though not too much, heads that flow some air, chamber volume to the small side to build some compression; you didn't say what your current intake is.......if not an Air-gap, then move up to one, 750 cfm carb, headers, you're there. May, maybe, need a converter change to work with what ever cam you go for.

Good luck.