The gasket supplied by Mitsubishi wasn't quite right, very close, but not quite right. I had to make another hole for a dowel, and move a hole for a bolt. Hopefully the instant gasket will seal that second adjustment. The gearbox now turns freely by hand and even coasts for a short time. The gasket was about 0.6mm thick and the differential became quite loose. I re-shimmed it to about 0.12mm (specified for 0-0.15mm) using a dial gauge and an old piston ring. I'm not sure why Mitsubishi call for the "fuse" crushing technique there as it's possible to measure the play through the driveshaft holes.
I didn't attempt to measure the preload on the shafts inside the gearbox -- I'll do that when I take it apart to remove the gears I don't need. John suggested that I could use lead wire as the crush "fuse", as pure lead is very soft. I'm open to other materials, but lead does sound like a good option.