ANY rebuild or restoration can look and become overwhelming. You will have moments of great satisfaction and then utter frustration just wanting to throw in the towel. This is part of any project.
Rule No1. DO NOT SET A TIME FRAME FOR COMPLETION. Its done when its done, whether 1 year or 20.
Rule No2. Do not try to adhere to a strict budget. Go ballpark with all your numbers because prices change, shop rates change, parts you thought you weren't going to need you now do, and some parts you'll just decide to add as you go because the once stock parts seemed good, but you want something more modern or because it is an improvement.
Rule No3. Don't add up all your assumed costs for a complete rebuild and say I can't afford it. Break your build & costs down into blocks, ie engine, trans, front suspension, rear suspension, brakes, body, interior, etc.. Build as you go and when your cash supply runs out, you save up for the next phase - and know it is an ongoing phase, and not a stopping or quitting point (Pontiac'ers are not quitters nor do we ever stop, we endure and eat macaroni and cheese for long periods of time).
Rule No4. The investment in your ride is more than likely going to be less than buying a new heap from Detroit -which depreciates like a stone the minute you set your butt in the drivers seat. Done right, your car is an investment. Even if you are upside down on your costs to rebuild versus resale value, you will still be able to get more of your cash back should you sell it than you will out of that new car that is now 10 years old.
Rule No5. Build the car YOUR way. You don't have to restore. You can alter or change things based on your tastes and budget. Make it the car you see it as being as you drive it down the road. Maybe you do like I did with my '68 Lemans. Rebuild the undercarriage with new parts & make it safe, rebuild a mild HP engine/trans, shoot a coat of primer on it and enjoy the heck out of it -knowing that when the engine gives up or gets tired after all those years of fun, you are going to do rebuild #2 when you are a tad bit older, and more financially secure so you can go all out. Maybe a Dad & children project when they are older and can help and appreciate the experience?
Rule No6. If you get rid of the car, will you be able to afford or get another? Most of us here can attest to letting cars go only to kick ourselves later because now we can't find "that" car let alone afford it at current prices. So if you are overwhelmed - store it and come back to it in 5 years and see how you feel about your car/project.
Rule No7. GTO's are great collectables, BUT they were meant to be driven and enjoyed whether in rough "rat rod" condition or a 100 point restoration - and you'll always get lookers and thumbs up wherever you go in whatever condition your car happens to be in.