307 vs 308 bearing retainer

Ok back up the bus;
What hole size is in your bell? and
what bearing is in your trans?

The standard A-body bell has a 4.35 hole, and there is no way to put a 308-bearing retainer into it, which come in two flavors; 5.125 and 4.80.
AFAIK there is only one combo that cannot be matched which is the smallest hole (4.35) to the biggest retainer(5.125); both would have to be machined. ...And
The big bearing 308 case, cannot accommodate the small 4.35 retainer; there is just not enough physical room.
also AFAIK, the overdrive box always comes with the big 308 bearing. But you have two choices of retainers to use with it; namely the large (5.125) and the medium(4.80).

The sweetest combo is the 5.125 big-hole bell; you can put any retainer into it , just by fabbing spacer rings. And the best part is that they cost peanuts. However; it can be tricky fitting that bell into your A-body, with the various Z-bar anchors, forks, and fork pivots. The parts are all available, you just have to choose carefully.