Drenth, Tractive, Sellholm, Quaife or Elite all make good gearboxes, some take power better than others but you don't need to fit a 10k gearbox on a 400-450hp engine

I know of someone who may be orgainzing a bulk order of the Quaife 60g box which from what I'm told it a very good piece of kit.

http://www.quaife.co.uk/60G-6-speed-...HT-CUT-gearbox

Prices will be very good