[special-K]

Those look like 16.5 wheels, but probably 16". I see 235/75s a lot on stock wheels, as as a radial equivalent. So go from there. I never looked to see if 85 series are made for 15s. In 16" that's what I look for to get tall/skinny. If not tall 15s can be found you could try to find the 6-lug 16 factory wheels. Could get lucky, pretty hard to find. Here are 255/85-16s on 7" wheels (would work with 6") and 4" lift (less in rear) on the K2500 to give reference. They're about 33". I'm sure they would work with 2-2 1/2". Probably would work with stock height. Next size down would be 235/85 which is closest to stock bias 7.50-16s, the size 3/4t and 1t trucks went to from factory once radials came along. I would judge those would be close to the diameter shown in your picture. They're around 31"