If you was to really clean them up, you may find a date code stamped somewhere on the wheel, other that, it's just a guess. To tell if they are road worthy, you would have to dismount a couple and look at the inside bead area and the back sides. Wheels are fairly thick steel and can take some rusting without much danger of coming apart. Powder coating companies will bead blast and coat the wheels in many colors for a reasonable price, so it might be worth while to have them cleaned, inspected and recoated, probably for less then the $100.
Tire size is up to the truck owner, 235/75 is pretty big for a steer tire, but works well for the rear of most any truck. OEM tires are narrow and tall, which make for better driving speeds due to the hight and rear axle gear ratio. Tall skinny tires will steer easier then shorter fat tires, same with air pressure, more makes for easier turning at slow speeds.
Have you been here and looked at the GM information packet on this truck?https://www.gmheritagecenter.com/gm-heritage-archive/vehicle-information-kits.html
It will help a bunch when speaking information.
To tell if a wheel is spaced correctly, check for brake drum/rotor clearance, tie-rod clearance at full left and right turn, frame and/or fender clearance in the rear.
Joe
