I have a TH-350C that had a lot of leaks when I got it. The biggest leaks were from the "bullet" that the speedo cable hooks to and the weep hole in the back of the yoke. Neither was terribly hard to fix.
New seal and o-ring for the bullet was just a couple of dollars, but hard to locate. I probably should have just gone straight to the dealer. I followed the directions for fixing the yoke leak in 73-87.com. I got the o-rings for it at the dealer. All of the auto parts stores tried selling me stuff that was just plain wrong. They must all use the same set of books or something.
It kept developing leaks, and I kept chasing them down. I finally ended up buying a complete seal set for it transmission. It was only about $30, and includes everything. No more parts to chase down.
I just replaced everything I could figure out, and that worked for a little while. Eventually the seal at the pump started leaking, and I had a transmission guy fix that using the leftover seals. I should have just had him rebuild it while it wa out. It is now leaking around the torque converter.
Slonaker