Do you have 15" 8 lug wheels? That wasn't very common in my experience. Of the (4) 70-72 3/4 ton trucks I have owned they all had either 16" or 16.5" rims.
But you will need to buy new wheels if you have 15's. You can run some 15's but they usually require minimum backspacing and look goofy IMO. FWIW, the disc conversion on my 70 GMC C2500 was the best modification I did to the truck.
Search here on the board for C20 disc conversion. Here is a link to search I did.
https://www.google.com/search?domain...D%3A1%3B&hl=en