Can't help with the SD specific questions. Symptoms you just described could be the starter, but it could also be wiring and loose connection issues. Did you check the heavy gauge wires on the terminal block and also the connections on the starter motor? On my 85, I peeled back the heat shrink on the crimps of those heavy gauge wires, wire brushed the corrosion away then soldered the crimps to restore good connections, have not had starting problems since.

The heavy gauge wires on the terminal block looked clean when I went to jump them. I'll check the connection at the starter.

To see if they are clean or not, you have to remove the wires on the terminal block, cut away the heat shrink that is covering the crimp before you can see if there is corrosion under the heat shrink.

I replaced the starter this afternoon with the spare I had from my old 617 engine. The good news is that the car starts on the first bump, no hesitation. The bad news is that it appears as if the starter gear is disengaging slowly because there is a buzzing sound coming from the engine bay right after start up. It barely lasts a second and isn't very loud, but it's obviously not good. I didn't have this issue when the starter was on the other motor, so I'm hoping that things will loosen up after 5 or 10 more starts. Otherwise, I'll get the starter that I pulled off of the car today rebuilt locally.

You didn't bench test the starter before installing it? Just power it up with a battery and listen is all that's needed if you have a trained ear (like a starter rebuild shop tech with 30 years experience). If you do not have a trained ear, use a photo tach.

On the starter you pulled out, bench test and shoot a video and post it. Take it apart to inspect, replace the 2 bushings (they're cheap), clean and lube, replace brushes if worn. That's all that's needed in a rebuild if the parts (armature, solenoid, centrifugal clutch) are good, which they usually are.

Routine Starter Service
 

That was my thought too ~

At the very least, have the bad starter rebuilt and save the working one in there now for a spare....

Never, EVER accept any BOSCH brand starter in exchange that doesn't have a brandy new solenoid ! you can always tell by looking at the solenoid closely .