[Kevin Sevcik]

Given my experiences with the P60 gearboxes, I'd agree your most likely problem is that the pinion isn't pressed far enough and it's binding when you reassemble the transmission. Loosening the 4 long screws will help determine that as long as you keep everything aligned while you do so. Other possibilities:

You bent your motor shaft pressing the pinion on.
You've forgotten the shim plate between the last stage and the motor mount block. The gears running over the screw holes and add some resistance, though it'd likely only freeze a very high ratio.