Looks nice. First thing I would do is get a copy of the cars invoice from PHS document services.
A GTO with drum brakes? I find that very odd. Disc brakes were standard on the GTO package, optional on the Lemans. You are sure you have a GTO (242-first 3 numbers on your vin) and not a T-37, Lemans, or Lemans Sport(233, 235, 237)? You could get the GTO nose on other bodies.
Historical value? In my opinion, the minute you put any replacement patches or panels on the car, it isn't original, even if you install NOS panels. The NOS panels will of course be the exact replacement in contour and fit as opposed to reproduction stuff which is typically close, but not 100%. And, you could get a NOS panel and have an idiot replace it and do a crappy job rendering that perfect NOS piece worth far less -so you have to have a good installer. Do you have to install a complete quarter as opposed to patch panel repair?
If you like the vinyl, keep it. When you replace it, there is a tendency for rust to develop around the rear glass, so a good time to check for this & repair if needed. Most restoration parts catalogs offer vinyl tops in colors to fit.
Exhaust, aftermarket won't hurt it unless you are lookinfg to go 100% concourse restoration -and not drive it much so it does not rot out again. If you can afford stainless, its a good choice. Get duals with the X-crossover pipe. Pipe size is based on your engine HP. Typically 2.5" is good.
Disc brake upgrade is a no brainer even if it did not come from the factory this way. Most of us that have drums do this for safety sake -better and more reliable braking.
TRW pistons are simply a throw back from the "old days", everyone seemed to have them. You don't need forged pistons unless you are going to build up the engine over stock, ie RPM the engine more than 5600 RPM's. Cast pistons are fine and cheaper -if you need them. Now we all will tell you to upgrade your stock cast connecting rods to the aftermarket forged rods that look as factory if you rebuild the engine. They are stronger, more reliable, and insurance. For the price it will cost to rebuild your stock cast rods, a few bucks more will get you better rods. Factory rods are indeed fine if you stick to factory RPMs of your engine and if you want to keep it original. You will need a good machine shop to advise you what you need with regards to worn/replacement parts.
Do your research and make yourself knowledgeable on your car and its year. Get a few books on your year/model. Ames offers chassis manuals and the like. Good to have these. Ask a lot of questions.