Welcome to the bolt! I'm partial to the '59 model! The 31 means its a model 3100 Short Wheel Base. Mine is a 32 (3200 long wheel base).
Finding original solid doors will be difficult and aftermarket doors are pricey!
If your door bottoms are rusted out (those doors are notorious for that) but the outer skin is good, you could buy some patch panels like I did for mine and cut out the bad metal of your door and weld these in. They are about $60 apiece and you can use your doors!
I bought a flux core welder from Harbor Freight to do mine and it turned out pretty well... plus I'd never welded a day in my life prior to that.
Congrats on the project!
Jerry
