['68OrangeSunshine]

Hi Brian,
[That's my first name, too.] I have a '68 C/10 Stepside that originally came with 6-lug 17.5'' x 5.25'' split rims. I wore mine out after about 250,000 miles. I had acquired one Firestone rim in that size, but structuraly different from the GM rims. Different hubcap clips. That was in the late '70s. When the tire shop said my OEM rims were shot, I tried to get replacements, new or used, in the 17.5 size and came up short. It was a strange rim size. I was told municipalities and corporations used the 17.5 size on fleet vehicles to keep employees from stealing tires. One board vendor had a 17.5 rim and 7.00 bias tire for sale, but he wouldn't dismount the rotten vintage tire from the rim, and I thought shipping rates from Washington to Tucson were more than the rim was worth, so I passed.
Original tires were 7.00x17.5 bias. I went to 8.00x17.5 in the mid '70s when they became scarce. I liked the "lift" but my speedo read slower.
Bias ply tires became obsolete in the '80s, so I changed to radials: 8R17.5LT.
I'm now running 16'' white spokes with LT235/85R16 to get about the same ride height as the 8.00 x 17.5 [8R17.5LT] I was used to.
If you do find a source for new, or good-used 17.5 x 5.25 rims in 6-lug 5.5 spacing, let me know.

If you just need tires, I had good luck with Yokohama Super Steel 703B, they were kinda knobby. Also Bridgestone V-Steel Rib 265.
Hope this helped,
Brian