Help me build my 340

A thorough rebuild on the stock 340 is all I would do. Have hardened seats put in for todays gas. That car is light and that 340 rebuilt, timed and tuned properly will boogie boogie!!! You might find 3.91's not very highway friendly. If you plan on cruising at highway speeds you might consider 3.23's, or maybe even 2.94's. It will take away some low end punch, but gas mileage will go up and the rpm's will go down at 55 mph. Should you ever decide to do a performance build, I would grab a 360 for the making, and leave the original motor intact and stock, and unmolested.