Must be doing something wrong... just had TH350 rebuilt and have installed onto fresh motor. Hooked up linkage and can't locate Park, Reverse, Neutral and so on. Does the motor have to start and the trans fill with fluid before anything works or did I get a bad rebuild?

Not sure. I myself have the TH350 and i hope when i rebuilt i don't have that issue. Sounds like you got a bad rebuilt for sure. All should be working ? It's probably not connected to the engine right.

If you can't feel a click in gears, sounds like they forgot to connect the manual valve.

I feel a click in the gearbox but no reaction at the rear wheels... if I hit Park wouldn't you think the wheel would stop rotating?

If your question is does the trans, work with no fluid and engine not running. The answer
is no, but in park one wheel can turn if you do not have a posi of some type.

Remove your linkage and try shifting it by hand to see if you have park and all the detents seem right.