Early 904 rebuild

You guys have helped a lot with all of the questions I’ve had. Seems like when I get one thing fixed something else pops up. Recently I noticed a pinhole in the pan of my transmission. Instead of just trying to fix that I think I’m going to pull the transmission out and rebuild it or have it rebuilt. It’s an early 904 from a 1965 dart. It’s a cable shift unit. I think this was one year only. If I’m going to rebuild it are there any options I have as far as changing this. What else should I upgrade or change? Currently has a 340 in front of it and an eight and three-quarter rear end with a posi 3.22. I think there’s also the possibility of picking up a small block 4speed tranny. If it’s going to cost the same amount to rebuild the 904 would it just be better to get the four-speed?