Am I Crazy to Try This?

In talking with a transmission shop, the likely cause that my car will not move is that I damaged the front trans pump while trying to install the new motor. They guess it will run me about $400 to replace the pump and other gaskets and seals. My question is this: I've never torn down an automatic transmission. It's a 904 push-button. How hard would it be to go thru this thing? It seemed to shift well before I pulled and rebuilt the engine, the fluid coming out was pretty clean and red.

My credentials: I built this 273, have built other engines: a 440, an MGB, Opel, but never opened a tranny before.

Should I tackle this or pay someone else to do it?