Thanks for the help.
I have no idea what this rear end was in originally. It may very well be the one this 75 came with. There is rust on the vin tag making it very difficult to read. I'll keep researching.
Maybe the answer to your question could be as simple as calculating the ratio. I am sure there are many articles on the internet. It is easily calculated by rotating the wheels a certain number of times while observing the number of rotations of the driveshaft. Good Luck.