Assuming it is a 3100, the
GM Vehicle Information Kit says it has 6.00-16 tires originally.
Those would be ~28.4" tall, so that gives you a starting point.
Depending on your brakes, you probably need to go with a 16" rim to clear them. Given that 16" was the stock rim size, that should look good too.
What I did for my 58 was decide on the rim diameter, find a tire size for that rim diameter that gave me the height I wanted, then looked at rims available in the diameter/width that would fit that tire size until I found something I liked.
In your case, 215/75R16 is 28.7" tall and a fairly common size. If you were to pick that size, your rims would need to be between 5.5" and 7" wide. With that info, you can go to Summit, or Tire Rack, or any number of places and just look for rims in that size range with the bolt pattern you need.
(Please note: that tire size is given as an example, you can go wider or narrower with appropriate profile changes to get whatever height you want)
One more thing that I would recommend is going to someplace like Discount Tire and putting in the tire size you are thinking about using to make sure there are options available. If there is only one or two models available now, you run the risk of the size going obsolete and being unable to replace them later.
Edit: it is really hard to go wrong with Torq Thrusts. If you have 11" rotors, a set of 15s will probably fit. If you have a Discount Tire nearby, you can order one rim and test fit it on the truck before putting a tire on it. If it won't clear the brakes and you have not mounted it, they will give you a full refund.
