55.2-59 700R4 Source for '56 GMC?

[1project2many]

I don't believe they ever made a B-O-P pattern 700 r4 case.

I do not believe so. I have a 700R4 training manual from my dealer days. I once posted to a forum that a 700R4 may be available with dual pattern bell housing based on a drawing in the manual. A Hydramatic plant manager on the forum quickly corrected me. "No BOP patterned 700 was built." The only non-SBC version is the 60 degree V6 version from 2.8 F cars and S10's. If you have to use an adapter, the thick adapter with proper converter support is the only way to go imo.