[HO455]

Your build sheet shows option R74. According to the 68 manual R74 is a 700-17, 8 ply bias, on/off road, tube type tire on 6 inch wheel. I would guess that the back spacing is right around 3 inches.

You need to take those wheels to a commercial truck tire shop to get them serviced. 99% of other tire shops will tell you that they cannot be used. Mostly due to their lack of proper equipment and their desire to sell you new rims. Replacement 17 rims won't be hard to find if you don't mind going to a 7 or 8 inch wide rim. Earlier model split rims gained a bad reputation but the technology advanced and they are still being used today on large trucks. A competent commercial truck tire shop will be able to inspect and determine if your wheels are safe to install new tires on. Corrosion can damage the rims.

My high clearance rims are 17.5's so tire selection is more limited but they aren't split rims either.

Those rims are hard to find if your looking for them.

Link to the GM heritage center 68 truck information kit.

https://www.gm.com/content/dam/compa...olet-Truck.pdf