What made it "not as smooth?" -- that it was in a wrecker? I drive a 13 spd Double OD behind a 400 Cummins on a daily basis. Short W/B tandem axle day-cabbed road tractor. I think it's pretty smooth. The Transmission is smooth. The *truck* is rough (springs, not air bags)

I never got an opportunity to drive Bill's truck (the one this thread is about) but with air bags and a long wheel base, I bet it was nice!
It's been 40 + yrs since I owned that truck, but mainly I recall the 13 spd had a lot more overdrive whine then the 20 spd that it replaced. I guess the best way to explain it is that it was a big , heavy duty truck with a primitive feel to it and installing that 13 spd didn't change that too much.
I wasn't trying to be negative or whatever. Just talking big trucks & posting ,or linking, to T & T's down in ravines just lead me down memory lane & my time in the HD wrecker business.
I'm sure your friends restoration job on the General was first class. It certainly looks that way in the photo shoot.