Firstly may I add my thanks for the work done here, this really is a fine replacement for a poor re-manufactured unit.
But I would like to add one thing. You need to slot the mounting holes in the cross member to fit the mount. If you do this there is very little left and there would be a big risk of them breaking through when in use. So I will be adding plate to each side of the hole where the mount goes through the cross member to ensure it doesn't break through.
I considered that exact point, but decided that if it was okay for the OEM mount themselves to have slots in them, then from an engineering point of view it would be equally fine if the crossmember was to also have slots in it. Even ignoring this fact, the crossmember is made from 3.5mm steel, and there is 5mm of steel left after elongating the holes. I think that you are adding unnecessary complexity, though of course that is completely your prerogative to do!
The clamping effect of the fastener is enough to keep them from moving, and even if for some reason the fastener had been insufficiently tightened, and bear in mind that I used new bolts and nyloc nuts, then the crossmember opening itself would restrict movement with a clonk to alert the driver of something untoward happening down there.
Checked the work at the weekend and I'm happy to say that after 520 miles of mixed fast road and town driving, nothing has shifted. Actually when I say nothing has shifted, my appreciation of the car certainly has, I see now why Alun said to me when I first joined the forum "...every mile in a Sprint is a pleasure!", because it has now been so, the car is transformed. I realise that seems over the top for just a gearbox mount, but then I'm admittedly a fussy one for noises and rattles, they just grate, and this is probably why I can't remember the last time I broke down, I hear them complain long before they give way!