Major TH350 problems

[fostrem]

So after a long haul of tracing my fuel lines, checking pressure and compression, and timing i finally got my old 76 shortbox to hold a solid idle. Now, unfortunately when the truck is in park and i rev the engine, it drives! I can only shift into park and neutral, or at least thats what I think I am in. I can dabble with a simple engine like my 350SBC but when it comes to transmissions I'm lost. Its got a TH350 in it. My plan is to drop the pan and see if theres any metal in it. What exactly should I be looking for? The fluid does look low and old so i will replace that. Does it sound like i should be in the market for a used replacement, or a rebuild? Any thoughts would be incredibly helpful.
Thanks a lot!