Tallest tire fitment on rear of Scamp

I am running 9"x28" slick on 6.5" wide wheels with 3.5" backspacing, no offset hangers. Inboard clearance to spring is.75". The tire was hitting the 1/4 panel hook on the bottom. Solution: move rear axel back 1". We are currently running 11:52 sec., 1.52 sec. 60 ft. Absolutely 1,000% street legal. Hooks like hell, goes like hell. IMO 275 radial would be a safe size. Good luck, Tom