I got a TH350 in my sons hot rod and it’s had a leaky trans since the day we put it in. Not sure exactly where the leak is coming from. It’s got an aftermarket tube and dipstick so for all I know maybe it doesn’t register the correct level and maybe it’s as simple as me putting too much fluid in. Looks like the level is right about the pan gasket level when cold when it stops leaking. There is a tube right behind the circulation lines that looks to be open. Not sure what this is( pic). Anybody know?
That's your throttle valve or "kick down cable" you're missing the end that attaches to the carb linkage, I've had several that leaked where the plastic housing goes in to the trans.
Like Duff, said it is the kickdown cable location. Someone cut the cable off to use as a plug. If it is leaking there pull it out and replace the little o-ring that should be on it, and make sure the end of the cut off tube is sealed with some silcone. As far as the fluid level, at one time I was told that the level should be close to where the dipsitck goes in ,with it ideling in park, I don't know if that is true or not. You can check where the full mark on the dipstick lines up on the tube on one that you know is correct. The other common places that leak is from the shift shaft, and speedo areas.