Many first-time restorers...
Many first-time restorers do not attempt a body-off restoration, and for good reason. A full frame-up restoration is at a level of complexity beyond most first-timers. However, the most thorough and highest-quality restorations are rotisserie restorations. The body is essentially mounted on a jig, where you media-blast, cut out the bad metal, install the good metal, and do all the finishing work necessary. A rotisserie goes a long way to speeding panel replacement. The ability to spin the body shell upside down speeds replacement of the rockers, quarters, and floorpan.
The initial disassembly is likely to be a straightforward affair, though inevitably there will be some slowdowns. Perhaps some bolts will strip or round out. Maybe some pieces will be rusted together and refuse to separate.
The best way to minimize those troubles is to use the correct tool for the job. That can be something as simple as using the correct-size screwdriver instead of whatever is close by. In addition, it means using a six-point socket instead of a sloppy adjustable wrench, or a trim tool instead of a screwdriver to remove a delicate windshield molding.
Fasteners, bolts, and nuts...
Fasteners, bolts, and nuts that haven’t moved in 40 years and have been exposed to road salt, grime, grease, and all types of corrosion don’t want to move. In many cases, it’s a battle to remove the body parts, suspension, brakes, and so on. During this process, penetrating oil and WD-40 are your friends, and you use a lot of them. Spray the fasteners 24 hours before you plan to start disassembly. Treat the fasteners just before you start. In cases where heat can be applied, use a propane torch so the threads break their bond and can be removed. A good penetrating oil goes a long way to freeing rusty fasteners and speeding up disassembly. I have had very good luck with Blaster brand penetrating oil, which comes in an aerosol can and can be purchased at most auto-parts stores.
Be sure to use a good penetrating oil on rusty bolts (such as exhaust manifold bolts or other engine fasteners), particularly if they have been subjected to heating and cooling cycles. Several good products are available at any auto parts store. Some restorers have their own recipes for penetrating oil, using automatic-transmission fluid and kerosene. I have had great success with a product called Blaster. I don’t know the exact formula, but it has worked better than anything I have used in the past, making easy work of loosening corroded and frozen engine fasteners.
You are sure to find some surprises during the disassembly process. With a car that is 40-plus years old, it is very likely that some areas will be in worse shape than expected. Old collision damage may reveal itself, as well as some sub-standard repairs. Hidden rust-outs on the inner body structure or frame may be revealed as well. Cracks can also be seen, particularly around engine mounts or suspension mounting points. Also, if your GTO spent time in the rust belt, don’t be surprised if your car has a good deal of body filler. Being from upstate New York, I have seen many cars that appeared to be straight but had substantial amounts of filler in them. The Northeast is full of skilled Bondo sculptors, so bring a magnet.
If your car came with a vinyl top, expect a significant amount of corrosion damage. It is pretty much inevitable that a vinyl-topped car from that era, GTO or otherwise, will need some rust repair, usually around windshield channels.
Unless you are planning on a correct as-built restoration, my recommendation is to remove the vinyl top, store the trim pieces in a safe place, and finish the car with a steel top. Alternately, the car could also be finished as a two-tone, using the vinyl-top trim. After going to all the trouble of repairing the roof, welding in new window channel, prepping, and painting, why set yourself up for another rust problem later on?
A body-off restoration is...
A body-off restoration is the most complete and thorough restoration for a GTO. When separating the body from the frame, you have full access to the framerails, chassis, and body. The body can be mounted to a rotisserie for media-blasting and painting. But most importantly, when the body is separated from the frame, you can cut out all the rusted panels and replace them with fresh sheetmetal for the highest level of restoration.
Body Off or On?
It’s true that body-off restorations are what you typically see in car magazines and at the large collector-car auctions. However, your decision to pull the body off the frame and perform a ground-up restoration should be dictated by the condition of the car, your individual situation, and the intended use—not by what everyone else does. Remember, you are the one doing the work (or most of it) and financing the project. You will also be reaping the rewards of your labor, so decide what is best for you and your GTO.
If you are restoring the car as a weekend driver, the “necessity” level drops considerably. If your car happens to be a rust-free example from the Southwest or is not in need of a significant amount of rust repair and/or weld-in panel replacement, skip separating the body from the frame and refurbish them as a single unit.