There are slant 6 bellhousings for them, and I'll shortly be making an adaptor to put them on the 6-cyl Hemi in Australia.
If you work with the plate type bell used in 1960, you could cut the bell part off that I'd reckon and use the plate, then adapt that to suit a small block bellhousing.
The overdrives started in 1974/5 and ran through to about '83, I think, maybe a year or two later. They're plentiful enough, lots of trucks had them, came in both A-body and B-body length.
