M'Lord, Welcome to the Team!
Tuning options are slim as you are at the apex of all of the wrong decisions that GM made trying to comply with uinleaded gas and emissions. Neither had been implemented in 1976 so the engineers were working on worst case fears and had guessed wrong pulling out too much timing, and running far too lean, with little to any compression.
So you had the lowest power rated motor in the heaviest car that GM ever made. To gain more power you need to replace the motor with one that fits the weight of your car. Chevy made the SBC 400 just for this application.
The 400 (bored thirty over to displace 406 cubes) is a big block in a small light weight package. With a new, more modern, RV cam designed for a heavy vehicle such as a medium duty truck, and a four barrel, it will make as much, if not more power than than the 1970 396 (which was actually displacing 402 cubes). They are found in pick-ups and full size cars from 1970 through 1978.
Your current electric controlled Quadrajet carb has plastic caps that prevent you adjusting the idle mixture away from factory settings (idle has no affect on power leave them alone) the distributor curve comes in far too late, and the carb is jetted far to lean. Your combustion chambers are 78 cc in size (in 1970 they were 64 cc), so you can not make any compression.
Compression is free horse power but it makes for higher nitrous compounds in the exhaust. At idle the EGR is open causing it to reburn old exhaust gas to improve emissions but ashes don't burn so it makes for a bad idle and hesitation.
You have a catalytic converter which is a plugged exhaust system because it is a can filled with ceramic beads which won't let the exhaust flow out quickly. Because of the catalytic converter and the room it takes up under the are there was no factory provision for a dual exhaust.
The electric carb I mentioned and your distributor are both controlled by the transmission (two years before the first OBD computer control) so that you get no power (retarded ignition and lean carb) except under a heavy load (vacuum controlled by-pass).
If you live in a state where there are no annual inspections (and even those that do exempt cars made before 1977) you can remove the cat and install a dual exhaust from an earlier 1971-'73 Impala. Swap the motor for a bigger displacement (the weight of the car is hard to change which is why Chevy abandoned this body style in 1977 reducing the size of the Impala to a mid size car) to move your Caddillac Brougham sized barge (which had a 500 cube motor) which when you put 350 decals on it looks like your current smog motor. Just without the EGR in the intake manifold, a Holley on top and you can actually see the motor with all of the vacuum lines removed (don't use a chromed water neck to replace your temperature controlled vacuum switch, as they leak). The 400 will pass emission checks you just have to change the decal on the radiator core support from a 350 to a 400 decal so that the person checking emissions plugs in the right test procedure for the added displacement.
I had this same engine in my 1976 Nova that I yanked and replaced it with a 454 to get better performance. In Florida there are no emission or safety checks so you can run a top fuel car on the street if you want to.