Looks like you are starting with a good body and paint (?). Given it was a 340 car, I would stick to finding a good running/documented rebuilt 340, vs putting in a 440 or 360 since 1972's didn't come with either. One word of caution on the 340---- they are fairly expensive given their rarity, but if you shop around, good deals can be found (just saw a complete 1970 340 on eBay tonight for $2k). Just be careful how much you spend (project plan list for parts & labor) given 1972 and up model years don't command the big bucks on resale and you don't want to be grossly backwards unless you have $ to burn.
Just saw a rebuilt 727 on eBay for $500, but unless they are within your driving range, probably best to shop your local craigslist.
Best, Charlie
70 Challenger
69 Camaro RS