Thsi is a little more work, but the oem tire specs can be found on many of the sites that sell reproduction tires, like Coker. Once you have the nominal diameter and width, you can compare them to the specs of pretty much any modern radial tire manufacturer and get a close match.
Since you are looking for tires for an AD body truck, though, the popular modern tire size that's pretty close and has the right look is the 215/85/16. It will work on the stock rim, even though it is a bit on the narrow side of the fitment spec. That size is pretty common for later model 1 ton dual wheel pickups, shouldn't be hard to find.
