Update. Not big, but it's something I needed to do that got done, finally.
Got the screws I needed to mate the GB to the motor. 10/32 3/4" cap head. It's amazing that something that costs 59 cents can stop an entire project for nearly a month...
Plate mounted to motor. Had to take those washers off, but you get the idea.
All assembled.
I also test-fitted everything (minus batteries, as I don't have them yet) to the rack, and they fit nicely.