I agree with 52Carl. Been a bit since I've been in the bottom of a 400 but iirc there will be a linkage running from the selector back to a tapered pin that forces the parking pawl up to engage one of the drums. this is held in place by a bracket and the pawl itself has a spring to retract when the pin is moved. I think the pivot pin for the pawl gets driven in from the external rear of the case and the hole is plugged with a small cup plug.

Not seeing many good pics in a google search. The only problem I could see running into trying to install the setup would be some of the holes not being drilled out.

Chris

Too many to work on
4 bolts, 2 square bodies, 4 GMT400s...and those are the keepers