Thought I'd have a go at mounting the gearbox last week.
I'm not sure I'm quite happy with what I've done, but I can't think of a better way...
After a lot of umming and arring I made something sort of like the main engine mounts, but smaller, and using a rubber engine mount from a classic/original mini.
First I tacked a plate onto the base of the tunnel with the mini mount attached:
Then I bolted a plate onto the top of the mini mount, and another on onto the bottom of the gearbox, and tacked in a couple of plates between the two.
Finally I removed it, added some sides and fully welded and this is what I ended up with:
And here it is back in the chassis:
Does that look like a vaguely sensible way to do it?