I have a '74 Grand Prix which I've owned for over 25 years. It is a good sturdy bike. The Weinmann center pulls are very good brakes. As mentioned, check Sheldon Brown's site for tips. My best suggestions when you start working on the brakes is to clean them and the cables very well, lightly lubricate, add some new KoolStop pads and switch the wheels to aluminum alloy rims if you can. Be patient when adjusting the brake cables and use a "third hand" brake tool. My bike is stored in the attic now, so I can't go check, but I think that frame is designed for centerpulls with a cable stop "bridge" on the rear and an integrated front reflector/brake cable mount as part of the headset spacer, so switching to sidepulls may be a hassle.
Best Wishes!