TH 400 Damaged

[cppursell]

My friend is experiencing some woes with his TH400 transmission.

He was putting in a new engine in his '64 Chevy C10. He hooked up the torque converter to the flywheel apart from his transmission (BTW he didn't fill the torque converter with fluid, IDK if that's important), then tried to bolt the transmission to the engine. While he was tightening down the transmission he said he felt resistance, but kept going, and he then heard a loud "pop". He took a quick look inside the transmission but didn't see anything broken, so he proceeded to finish installing.

Now my friend's truck is running, but the transmission won't engage. We both fear the transmission is ruined, but perhaps a part broke inside that can be (relatively) easily replaced. Could it be the fluid pump that broke? How easy is it to take these transmissions apart?

Thank you for your helpful responses.