Ok, I didn't realize it was just 4 screws, so I tried it myself.

Spur gear fits fine. I need to measure the exact clearance, but the spur does hang down a tad lower than the diff mount, so the recess in the chassis is needed.

The stock screws are too long without the spacer installed, and the spacer is 4mm thick, so I need to go buy some 4mm shorter screws.

A couple of other noteworthy observations. There was what looked like a thin coating of oil (or something similar) on the chassis, but the diff is bone dry (thankfully). Maybe it's from the bearings?

So I'll go get some shorter screws tomorrow (hopefully), bolt everything down and see how much clearance I have between the spur and recess in the chassis. I'll take pictures for sure. There is plenty of room for the motor.