head height limit is you must have at least 50mm clearance from your helmet to the roof bars, may affect the seats being moved outwards.
roof must be more than (I think) 700mm from the seat hollow to the roof.
forhead of driver or nav must me more than 300mm behind the front hoop.

These are the ones I can remember off the top of my head, and dont take it as gospel - your best bet is to grab the CAMS bible and make sure that you've covered all bases, but other than this I dont think there are any specific provisions.