[Andyman]

I have something kind of similar that I bought more than 20 years ago. It's a Mercury Marquis with a 429 engine. The 429 is almost the same as a 460. The crankshaft and pistons are different. On long trips I could get about 17 MPG. My engine had about 10.25 compression ratio and I'm sure that helped. After 1971 they lowered the compression. After I stopped driving it I discovered that my distributor advance springs were pretty tight and it didn't get much spark advance until it was up to about 70 MPH. I loosened them to allow more spark advance at low speed but I can't report on what effect it had on fuel economy since I don't have that car registered.

An overdrive transmission with lockup torque converter would be a big help on the highway when you have a large displacement engine. Something from a Ford truck might fit.