I agree with Bob C. If you're just treating it as a cruiser, nothing wrong with the stock 340 (after a rebuild, obviously). With nothing else as an option, the Pontiac 400 is an excellent choice for both a reliable driver, and a high performance engine. I had two '72 Pontiac Catalinas, one with a 400 2bbl that got me 20mpg, and lasted quite a while; and the other a 600 horsepower brute 406 that delivered 6mpg with my foot out of the secondaries (not very often!)
Personally, I'd either stick with the stock 340, or drop in a 401 or 425 Nailhead.
Either way, best of luck to you and your car!