You can google "tire size converter" and look for a diameter matching. Then, what I did was buy a used tire and mount it on the front and check for fit at the tie-rod ends.
It might be wise to get rims made for radials as the forces of radials are different....some say mounting radials to bias-made rims may cause failure.
I have bought Cragar Smoothie 15 inch rims and mounted 235-75-15 tires....Firestone 380 whitewalls, currently buy 3, get 4...they are about $74 each. My speedo is off by 4MPH at 50 now, but I intend to replace the rear gear later.
A friend is running 215-85-16's on his 1953....probably on the old rims.
