On my '73 Scamp, I fit a Hoosier 29 X 12 with a 15 X 10 rim with no problem. I did roll the top of wheel opening lip upward. I believe that Hoosier was a 13.7 section width, but you might get away with more but will be tight. That diameter tire gave me about 1/2 inch from the lower front wheel opening, and had to trim the front of the inner wheel due to clearance concerns. Also depends on how low or high you want the car to ride...the lower you go, the more ostructions there are to deal with.
I think the max amount of space you can get from the outer frame rail (with no undercoating
) to the outer quarter with the lip rolled up is pretty darn close to 15" on the Scamps, Darts, and Valiants I have measured.