David,
Q. Where does all the space come from in the GT40 ?.
A. Time And Relative Dimensions In Space /ubbthreads/images/graemlins/grin.gif
Yes, it can be difficult, not only diameter, but side clearance at the rear.
Most GT40's start life with 15" inch wheels. My 3 piece wheels have an internal diameter of 365 mm, that generally equates to a max disc size of 300mm. A lot of cars have 17" wheels in order to get larger discs in, however the car is way lighter than road cars, so 300 will do for most purposes.
The biggest problem is at the back. The weight distribution is such that rear calipers from front engined cars are too small for effective braking on a mid engined car. With sliding calipers, there is no room to get a 4 pot in, so you need a big wheel offset, or a repositioned caliper mounting lug.
Dave Bilyk