I'm not sure. There were adapters and bellhousings made by the aftermarket at the time to swap Y-Blocks into older vehicles while retaining the rest of the stock driveline. However an F5, being the more heavy duty truck that it is, would not have been the target of such hot rod parts. I don't know if those things would have worked for that.
If you go to eBay and search for Y-Block Bellhousing, you will see all of the Factory Y-Block offerings as well as an Offenhauser transmission adapter for '32-'50 transmissions. You can use those images to help id what you have. Another thing to do is to look for a model number or date code on the transmission but if it is all gunked up like most things that old are, it may not be easy to find.
Lawrenceville, GA