You gotta put a 409 in it! Ok, where do I send the $100 bill? HaHaHa. It is funny how certain makes/body styles conjure up a specific engine/combo. Say 1970 Chevelle SS and it has to have an LS6 454 4speed. '69 Plymouth RoadRunner and you think Hemi/4 speed. AMX and its 390CI/4 speed. Yet all these makes and others offered base line engines with far less horsepower, automatics, and stiff highway cruising gears. Nice to see you look forward to driving and enjoying the car more than having to save up a nest egg of cash in order to take it out once in a blue moon. The 409CI I had got 8-10 MPG. I myself in today's world, making more money than when younger, would be hard pressed to drive it regularly with gas prices as they are. I think I would have to trailer it to the shows and then drive it around - like a lot of owners do with low gas mileage/high HP cars.You already rec'd some good info here, but there's always the OEM selections too. Most were able to offer plenty of grunt with the stock convertors and gears. They spent millions developing an overall high performance car in most any combination. With a Turbo 400 you'd need a loose convertor for most of the hipo cams in the aftermarket. With the gear you plan to drop to you may end up with a lame combination even with a loose convertor, and to add insult to injury it would probably have an insatiable appetite for premium fuel and lead additives. I'm building a 61 Belair bubble top, and if I had a $100 bill for everyone that yells "...you gotta put a 409 in it.", I still couldn't afford the fuel!!