Space between cab bottom and frame?

[AirSpeed]

I am repainting my short/fleetside 72 GMC 4 wheel drive and it looks like the cab has sunk in the rear. I looked at the body mounts and they look like they have been recently replaced. If I raise the rear of the cab up to make the space between the bed and the rear of the cab parallel the body lines will be above the lines on the bed by about an inch. I can see a definite dip in the middle of the truck, it looks like it is bent down in the middle, The frame is perfectly straight. Am I going to have to raise the front of the bed and rear of the cab to get it straight? Has anyone ran into an issue like this before?
If one of you has a similar truck and wouldnt mind, could you measure the distance between the frame and the bottom of the cab where the rear body mount sits? I think this will give me a good starting point to figuring this out. Thanks for any suggestion! Aaron