If you're getting high rpm and poor mileage, you have a short gear, rather than a tall gear. 3.55:1 is a common choice for best of both worlds, but with the 9", you can go as high as 1.9:1, which was a factory option in the Lincoln Versailles.
I have to 100% disagree with the statement posted that modern transmissions are "made with stamped steel parts" and do not have the "nice machined parts" that old transmissions have. Todays transmissions have as many, if not more machined parts, often to far tighter tolerances than any trans ever put into a vintage vehicle.
Its obvious you have never taken one apart. The 700R4 is full of corrugated sheet metal welded to disks for clutch packs. In the old days it was all machined.
Go visit a transmission shop and ask to see them.
