The complete transmission would interchange. The preceding numbers are not referenced in the Hollander and that is about the only book that will tell you casting numbers. They used a 3/4" and a 23/32 shaft. I think the B is the small shaft and the C is the larger of the shaft. .7500 compared to .71875 should be easy to measure with a caliper. If you do have 2 different shaft sizes. you have a problem already with the top shaft. Your 53 may have the synchro with 4 dogs on it compared to just 2 dogs on the 51 model.