Need help with tire sizes

[68c-10streetrod]

I've got a 1968 C10 6 lug 2wd fleetside. Looking to lower the rear of the truck, but since the tires and wheels have been changed, I dunno what level of lowering springs to buy. It looks like the front has been lowered already. I know the stock ride height is 32" in front, and 34" in the back. I need to know what size tires this truck came out with, so I can add or deduct the current tire size. I found a spec sheet on the truck, and found these numbers for tires sizes. Front 7.75x15 and rear 17.5x17. I've looked at several tire size websites, but cannot get a comparison between the size of the originals, vs the size of the current. HELP?!?!?!?!