Don't think I've ever seen a small retainer OD. The frt. Brg was oversized I believe because it was going to be so heavily loaded in OD having to transfer the torque thru' the 3rd gear set & for long periods of time.
They made a few early ones with the 307 bearing. You just swap the front retainer for the standard 4 speed retainer. I have converted a couple O/D to standard main housing to fit in the 66 without mods. Does not seem to be a problem.
