What options for O/D behind a S/6?

Youse guys are doin' something wrong. I had a '76 Dart Lite that got 27-28 mpg at 70+ mph and 29 mpg at 65 mph. I think that car had a 2.76 rear. Al lthe 225 cars my family had without OD's (2 autos and 2 3-speed manuals) got in the 22-24 range on the interstate.

Probably won't be able to make it work, but the old Hone-O-Drive is back: http://hone-o-drive.com/

If you have no use for that 4 speed OD Feather Duster trannie, please let me know!