If you want to pull a boat faster, gearing is not going to help. You need more horse power. I have been fooling around with overdrives for over 50 years and I like them. Just about all of them were behind a Y-block. But the tail section is the same Borg-Warner that GM used. I wouldn't worry about model numbers. A lot of people who are trying to to sell transmissions are clueless as to what they have. If it came out of a 55 and later Chev car or pickup and has a BW R10 , it behind it, it will work. The T85 T86 and T89 overdrive are strictly Ford application. They can be adapted to work, but its not worth the effort behind a 235. The Saginaw was used for a couple of years in the late 60's for high HP applications. They are so scarce that I wouldn't recommend looking for one. I found one several years ago at a swap meet and the guy was clueless as to what he had.
If you find a transmission that looks like what you have now except for having a OD tailshaft it will work with the shift cover you now have. If you have a closed drive shaft, you will have to go to an open drive shaft. That will give you a 3.90 ratio which is a good ratio for a 235. When you are pulling your boat, lock it in direct. When you are cruising without your boat, let her shift into OD and enjoy a little more speed.
