Is there a small vent on the tail shaft housing or on the side of the main case with a tube connected which goes down? Vent on tail shaft housing is 26 spline late C-4 which is good and vent on main case side is 24 spline early C-4 which is junk.
64 - 69 was only 24 spline. 70 - 86 could be either 24 or 26. Going by the number you give it is a 1966 24 spline c-4. I would try to find one from a pre 77 bronco or f100. Those are the heavy duty c-4's with the dipstick in the pan and the bell bolting to the housing not the pump.
The 'B' servo has one of the smallest apply areas.I wouldn't use it.
Why do you say the 'A' is bad?.It actually has the same size apply piston as the 'R'.I have had good results with the 'A'+'R' though the 'H' is the hi-po one if you can get one..
The 'B' servo has a diam on the small section of 2.7" where the 'H' is 2.96".The larger section of the B +H are the same size of 3.785".The 'B' would work fine with the mod of removing the outer seal but..