I have been playing with BW ODs for almost 60 years. I have read over the years where an OD from a Saginaw 3 speed installed on a Saginaw 4 speed. I have a 3 speed Saginaw with OD and I have a Saginaw 4 speed. They both use the same front case, but I have never been interested enough in the swap to see what the end of the mainshaft on the 3 and 4 speed look like. I really like the BW front section much better than the Saginaw front section. Rather than making a conversion with a 4 speed I would get some 4.11 gears to keep the thing from bogging. Even a 3.90 should improve things. The best rear I ever had behind a Y-Block was a 4.27 out of a 56 Ford station wagon. Shift linkage for a 3 speed OD is difficult to make unless you get a shifter made for it. I still have a few NOS Hurst Mystery shifters that were built or OD. I think it would be a nightmare trying to work around the solenoid with a 4 speed. I would rethink the whole thing.
