I got my car back from Gearbox Express today with everything fixed, and I'm very happy with the outcome!

The box from Gorilla Industries is now in my car, with the Quaife LSD replacing the clutch style LSD that was in there before. I left the synchros alone, and they actually feel totally fine, maybe even better than my original box. This could mean that I had worn out synchros all along in my original box, but got used to it without realising. Or maybe the revised synchro design makes up for it somewhat.
Before anyone asks, I haven't put the Quaife LSD to the test yet, but I will soon.
In case anyone is wondering, the damage to my original gearbox was exactly what I expected...broken shift fork for 5th gear, and a crack in the end cover for the casing:
I'm so happy it's all sorted. Now I just have to wait for my new rims to arrive so I can move on to fitting those!