Good luck on that one. I am learning new stuff all the time here so it'll be interesting to read what others post here, but I think you are barking up the wrong tree.
As far as tires and their height for the 20 inch rims, even the 7.50 is usually a little more than 36 inches. Now, I'm talking "old truck" rims and tires here. Definitely tube type. Maybe the new custom styled 20 inchers would have a rim and (tubeless) tire that would work on the newer Chevy truck. I think some of them are quite large.
