While the tire size should physically fit in the fenders, you will have difficulty in finding wheels that will bolt up. Most, if not all, of the 20" rims on the market in the correct bolt pattern are meant for late model trucks, whcih have the incorrect backspace to work on the early trucks. You'll either be looking at adapters or custom backspace wheels in order to fit them.
A couple words of of caution, you will lose ride qualtiy and load capacity with that size tire. They are very stiff due to the short sidewall. They are also not load rated due to the primary use as a passenger type tire.
