Hey, ive not seen more than 20.5mm spacers so i'd guess they'd have to be custom made? I think someone on here made some to sell, not sure how thick tho!? :|
I reckon you could go to 40mm but you would really need to use high strength bolts, and possibly support the spacers with lugs welded to the frame to make sure they did not move. I have 30mm on mine and as I say there is enough space for more..