Nice car, but they are boats and a bit heavy -over 4,000 pounds. I don't know exactly which year had what, but Pontiac began to install all the crash protection pieces on the cars to meet Federal crash standards, like heavy crash bars inside the doors, shock absorbing bumpers, double panel roof, and a front frame that will buckle/collapse when hit.
I have a 1977 2-DR Pontiac Lemans (and I think it was even heavier with all the crash protection built in by then) that I learned a very costly lesson with the hard way. Has the weight of a station wagon and took a lot of pedal to get it rolling at each stop -so gas mileage around town was horrible with the 350/automatic. Had 2.76 rear gears, but up on the highway at steady speeds, I could get a best of 19 MPG's after I installed a Q-jet, open element air cleaner, and tuned it up (single exhaust).
I totaled the front out (and have seen many in junkyards, bought a '77 Can-Am and '75 Lemans in the same condition). The disc/drum brakes work fine and will lock up your tires in a panic stop........and that is the problem. Your tires will lock up
, and the car will keep sliding like I learned - right into the car in front of me. I still own the car 20 years later, but it is sad looking now. Never have found a front end locally in the junk yards that was not hit like mine. Bent the frame just like it was designed to do too. Now you just can't find them period.
Trust me on this.
Your first investment should be in some wider than stock rims so you can mount wider tires. (factory seems to be the 15x7 with G60 tires) Get a good compound that has a good braking rating. You may not think you will ever have to do a panic stop, but it will be too late when you nail the guy in front of you -even if it is his fault.
Don't know what you have for an engine, but to move all that car around, you need a real torque motor. If you don't have a good torque engine, then headers, in my opinion, would not be where I would spend money and time trying to fit them. Factory exhaust manifolds, dual exhaust with 2 1/2" pipes, and free flowing mufflers would most likely the route I would go.