[Abomb]

I'm new to this particular forum, but I'm a long time Gmc/chevy 1/2 ton truck guy and recently bought my first big truck. Brought it down from kansas and it has a pretty clean cab on it with a later model v8 and 4 speed previous owners swapped into it plus a 2 speed rear axle and tilt bed. My goal is to fix it up enough to use it for actual work like hauling some dirt and steel. First thing I'm working on soon is the brake system. The stock booster is dead and I'm going to upgrade to a modern booster and cylinder, replace any lines if needed, wheel cylinders, shoes, clean and paint, maintenance, etc. until the system works and I can start driving it around town on short trips. At the same time, I need tires for it and I'm wondering if any of you guys know the best route to take. It has 8.25-20 E-rated tires with tubes and the 6 lug stock steel wheels with lock rings. The tires are at least 20 years old but they all hold air and maybe I can use them for a while since I won't be maxing out the loads or driving on major highways at all. 50mph at the most. I have called around to several tire shops for 20 inch commercial tires and i get the impression it is not popular these days in that size. Do I really need to convert to 22.5 wheels and tires to get the best bang for the buck and availability of used tires? Not trying to spend 2k on wheels/tires, but I just haven't been in the big truck world until now so I'm not familiar with options.