I agree with Denny, a V6 is a good conversion. I know how bad the gas mileage is in the stovebolt six and a V6 does have plenty of power. Another idea for engines in the AD would be a VW diesel, small enough to fit in there and strong enough to pull. Also the VW diesel is great on mileage; my 02 beetle gets 44/48 mpg. Of course the beetle is a little lighter than the AD but still...
Another point is, the VW diesel will outlast in mileage most engines; as they say, "that engine will still be running when the body has rusted out."

~Jim