WHAT SIZE DRAG RADIAL SS SPRING '65 DART

Come spring, an index class starts and car must use drag radial. At this point I am using a 7" slick on a 14" rallye wheel. Want to move up to a 15" wheel. How big of a drag radial can I fit w/o moving the springs and using mini tubs?
My '66 Barracuda will take a 235x60x15 on a 7" wheel pretty easily.