Wow, that's a lot better than I expected it to be when it was first described!
About the power upgrades...
Those cars, when new, made about 200hp stock with about 320 lb ft of torque. They are heavy cars, and most of them got lazy gearing. Yours most likely has the 3.23's with limited slip since it's a manual transmission car, which is as good as it got back then. For engine mods, there are a few things you can do that will appear close to stock:
-Exhaust:
Ram Air Restorations makes some really cool exhaust manifolds that will appear close to stock and won't be as annoying as long tube headers (ask me how I know). These are modeled after the old Super Duty manifolds from the early 60's and flow better than the stock log manifolds. Behind those, I'd go for a Pypes Performance exhaust. They make some really nice kits, and have muffler options from mellow to obnoxious.
-Cylinder heads: The heads that came on the 1976 cars are not great; big chambers, small valves, and worse flow than others. Pontiac made a ton of different cylinder heads over the years, and a lot of the good ones have gotten pricey due to scarcity. The best bang for the buck heads are probably the 6X4 heads that came on W72-equipped 1977-79 400's and some Pontiac 350's. This site from the dawn of the internet is thankfully still up and is a great repository of info on ID'ing heads: http://www.wallaceracing.com/head1.htm
-Intake and Carb: The stock intake is just ok but serviceable. A lot of guys swap on a Edelbrock Performer intake. The carb on that should be a Quadrajet, and can be rebuilt and re-jetted for better performance. You need to be extremely careful with changing out intakes and carbs because of the shaker hood. While some intakes do clear the hood (Edelbrock Performer is one of them), they only clear the hood with the stock carb. There used to be a guy named Cliff Ruggles who was awesome with Q-jets and even wrote a book on rebuilding them for performance applications. EDIT: He's got his own site now: https://cliffshighperformance.com/
-Camshaft and valvetrain: The stock cam is on the weak side, and a slightly spicier one will really wake it up. You could also switch to 1.65 roller-tip rockers, but you may have to get different valve covers. For the sake of originality, I'd stick with stamped steel ones that fit under the stock valve covers, unless you get some spacers.
If it were mine, I would stick to the basic stuff. Do the exhaust stuff, get the carb rebuilt, tune-up with a distributor re-curve, and go from there. I'd be weary of taking apart a low mileage, all original example, but it is your car. I regret taking mine all the way apart all the time. I haven't driven mine since since 2012!
