What you need to do is measure the setback of the wheel. With the wheel laying face-down, place a straightedge across the rim and measure to the mounting face of the wheel (the flat that goes against the brake drum). Also measure from the mounting face to the outer surface of the tire. Then with the tire on the truck (not on jacks) measure the distance between the tire and the tie rod end.
Ideally, you should have 1" clearance to the tie rod, some have gone as low as 1/2" and have had only occaisional rubbing.
Using these dimensions, you can sketch out what you have and combine them with the width of the tire you want, and figure the setback of the wheel you need.
What you are going to run into is that most of the newer wheels have too much setback for use on the older trucks. One of the big disadvantages of the older 3/4 ton trucks. One option is to go to the spoked wheels, sold for trailers, where you can get closer to the setback you want. Another option is to have your wheel centers mounted on newer rims, if you can find a wheel shop that does the work.
Wheel spacers are a love/hate relationship. I have no problems with them on the rear axle, because the wheel is loaded through the hub, which sticks out far enough to use fat spacers. On the front axle, you lose the load bearing surface unless the spacer is machined to slide over the hub and nest inside the wheel center. You will also have to install longer studs, they didn't make the originals any longer than they had to.
