cb's dodge

do any of you guys know the biggest size tires you can fit on a 96 cab and chassis? i know the rear axle is more narrow and a regular 3500, i tried to put 35's on it yesterday and it wouldn't fit it hit the rear leaf springs, the front tires fit with my leveling kit, plus i got the 2 inch spacers for the rear tires so what i need to know it how big can i go? i don't anyone who has had this problem thanks!