Welcome to the Bolt' Robin! I would say go the SBC route. The options and availability of parts for them can't be beat! You can easily achieve milage in the low 20's with a SBC V8 and an overdive capable trans. My brother has a 350 bored .80 over with a quadrajet, backed by a Camaro trans with 2 overdrives and a 3.73 rear end. He get's good milage cruising around. Just some thought.
