What does everyone reckon is a reasonable gap between the gearbox and any crossmembers/uprights?

Cheers

Depends how stiff the engine and gearbox mountings are --- Sierra mountings are very soft.

half inch is a general guide i think. but as above, depends on how much things can move

Rather rough and ready, but if you can half the gap giving it a really good kick....

4x landrover mounts like these:

http://cgi.ebay.co.uk/Land-Rover-Range-Rover-V8-Engine-Mount-Mounting-Rubbers_W0QQitemZ270424000075QQcmdZViewItemQQptZUK_CarsParts_Vehicles_CarParts_S M?hash=item3ef686c64b

I could possibly put in a top brace as well?

I use those mounts, and they are pretty solid.

I would go for around 10mm clearance. This is what I will be aiming for around everything when I fit the duratec.