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It's hard to tell if it's a GMC or not without seeing the dash. Also the VIN will tell you. That frame is either a K5 frame or a shortened Suburban frame. That box has been shortened also. It's hard to say on the front springs. They could be aftermarket lift springs with a leaf or two removed. I'm sure a good amount of the lift is from the body lift that is necessary to mount one of these bodies onto a '73-'87 frame.