Since you already have a locked up 318 in the car all you need is a 318 from 68 (LA Style / Post Poly Head 318) to late 80's early 90's (Pre Magnum) engine.

Swap over everything that doesn't match like the oil pan, intake, water pump etc.

Like people already mentioned the later 318's may have air injection ports in the heads. If the exhaust manifolds do not cover them, tap and plug them.

You can swap in a magnum but the intake manifold bolts go straight down and not angled to the face of the head. They require a different intake but even magnums have the ears on the block to bolt the old style motor mounts. Magnums will also take the old school distributors, oil pans, etc.

If you stick to a 318/5.2 motor use your existing flexplate and torque convertor. 360's require a different oil pan, an externally balanced flexplate, and motor mount bracket on one side.

Lots of good help here if you run into a snag.