Dave, I had these guys do some custom work on my clutch and so far it seems to be going really well.
Clutches - International supplier clutch kits, clutch parts, clutch plates, clutch cylinder
The Audi box is going to be your fuse as Chris suggests. My mate Rossco Miller in Vic has a RF GT40 and he's broken more boxes than I've had hot breakfasts. His clutch is still going strong though!!!
I think the Porsche G50 Trans (?) is the go for boxes in 40's isn't it.