727 trans wont stay shift into park

the other day I backed my Valiant out of the garage, put it into park, and got out to close the garage door. I turned around and it was rolling down the alley. I was able to jump in and stop it just before it went through the fence across the alley! the transmission is a 727, and it won't shift into park. It's a new project and I havn't been through it yet and I don't know much, or anything about transmissions. How difficult is it to rebuild a 727? Is it best left to a pro mechanic? I figured if it does need a rebuild, I would beef it up a bit.

Any suggestions or advice?