Cunning gearbox mount plan...

[mark]

Its a good point and a good solution, but in engineering practice if the bolt ever came loose it would fall out. thats why whenever you can put bolts in facing down so if the nut comes loose the bolt would still be in like a pin

Thats in theory anyway

But I would be quite confident in the mount you have described but would put a glug of loctite on the bolts for good measure