GM made three fenders for this era truck. Two different types for larger trucks, a small opening and a larger opening which is the one shown on the cab over. The third fits pickups and has a small wheel opening. I have all three flavors and the big truck fenders will not fit the pickup trucks and the pickup fender will not fit the large trucks. I learned the hard way and once bought a NOS fender I thought would fit my 3/4 ton pickup. I painted it and had it looking perfect and went to put it on and no fit, so now I have a perfect NOS big truck fender with small opening and nothing to put it on.
You can replace your big truck large wheel opening fender you have now with a big truck small wheel opening fender on your cab over.
Hope that helps.
