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It all come down to money. First off I would look into how much money your going to put into that Toyota to keep it running. My guess is that your Toyota is going to be getting you to work, if that vehicle becomes disabled then your going to have trouble getting to work. So keep in mind the money you'll be using on gas, insurance, and random maintenance.
So your looking at a GTO? "The Great One" as the commercials called it, not a bad car at all! Most car experts actually say the GTO was the first real Muscle car. One thing to keep in mind when buying a used Muscle car is that your buying someone else problems.
After coming back from Air Force ROTC training, my parents told me that I would have to find a car to get me to college. So after looking around town I found a car that had caught my eye. It was an 2006 Ford Mustang Convertible. As a college student making $450 a month from the military, $264 goes to my car payment. Well of course right away I started modding the engine. I put a Cold air take in, I bought a computer programer and re-tuned engine, changed up the transmission shifting, fuel grade, ect. I had almost spent $1000 on engine mods, to the point that the only money left in my account was for the car payment. 2 months later I was driving on a hot day and as luck has it, the temp gauge failed and I burnt out a hose in the middle of no where. I had to pay $500 to get it fixed, with the help of my parents of course.
Basically what I am saying is don't put all your eggs in one basket. Buying a descent shape GTO isn't going to be cheap. So if you spend all your money on fix your GTO and your Toyota fails, your going to be stuck in a tough spot.
Also don't buy a convertible, this was my first convertible and it will probably be my last. Its not that they don't seal well(mine seals fine, almost like a hard top), its just so much maintenance problems that go with it. They can be fun don't get me wrong, but it comes at a price!

If you going to pick a muscle car, go pre 1970's. The 70's had major fuel crisis's, which led to government regulations engine performance. Which basically killed the muscle cars performance.
The big issue that will also be present is your wanting a Pontiac. Pontiac has been out of business for a few years now, not to mention the last Tempest came out in the late 70's. Unlike people like me who own a in production muscle car, like my Mustang, your going to have trouble finding parts. With my Mustang, I just go to Americanmuscle.com to buy my after market parts for engine performance or body kits. Also I can just stop buy a local Ford dealer to fix major issues. The Tempest, however, has been in the market for over 20 years. Finding parts for that may, if not, even be impossible to find. I honestly couldn't tell you any one I know who owns a GTO, let a lone a Tempest. I know people who own Firebirds! Not saying that those parts aren't out there, but this car has been out of service for more then 20 years. Most of the junked cars or spare parts have probable been taken.
If you looking for the same body style you could always try Buick or Oldsmobile. The Ford Fairlane has the same style body, my GF's uncle drag races in his 1961 Fairlane. He has won a lot with that car.

Honestly since you have a plan you should be fine. You just have to make sure you have the resources on the parts. I am not trying to get to personal here, but are you still in High School?