First, don't freak out. Anything can be fixed, but you meed to find out exactly what is wrong. Can the frame be bent? Sure it can. Remember, the car is 43 years old. GTO's have a perimeter frame, and unless it is a vert, the frame is NOT boxed, except in the areas of the differential and the front end. The sides and the rear area are channel. The frame gains strength when attached to the body, but the sides and rear can be susceptible to twist. I have a '67 that I am re-doing now and I boxed my center section. Can the frame sag? Yes. I notice if I leave my car up on jackstands for a while, the door gaps will change, indicating a sag. All this doesn't mean that this is what is going on. Get under the car and look at the body mounts. If they are original, I can almost guarantee they need to be replaced. Mine were literally falling apart. Also the body mount holes in the frame tend to rust out over time as it is an area of entrapment for water. I repaired all of mine. The worst ones were the two just behind the rear wheels. They are brackets on the outboard of the frame rail on the weakest part of the frame. Take some measurements for yourself. Get a piece of angle and level it on the floor under the car and measure from it to the frame on each side at several locations. You are not looking for a specific dimension, you are looking for same side to side numbers. If you get very similar numbers from one side to the other from a level reference, then the frame is level in that area. You say that the rear passenger side is 3/4" higher. Higher than what? The frame, the ground, the tires? Measure between the body and frame at the mount locations. Make sure the mounts are tight. Sit down, if you drink have a beer or two and calmly think about what could cause your problem and how to fix it. If the frame IS bent, it CAN be fixed easy enough, you won't need a replacement.
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