[Elarson]

I'll cast my vote for the incorrect cam phasing and/or inaccurate ignition timing marks on the balancer.

Eric

[Formulajones]

I wouldn't go chasing exhaust. In Your case it's not an issue.

I ran Gardner exhaust on my RAIII in pure stock and ran 14 flat with it at 99 mph. While it wasn't holding the car back a ton it wasn't a pig either at a 3890 race weight

I did switch to a flowmaster transverse setup back then because 2 1/2" is legal and what most run in the class, and ran 13.70's at 102 mph with no other changes. Worth maybe 30 hp or so. Don't think that's your issue when you say it's a pig.

[Sun Tuned]

I know he said there was no issue with the secondaries.

Might not be running it through range of motion by hand, but I would highly suggest having someone in the car push accelerator to floor and visually verifying they open fully inside the car.

I’ve seen too many at work that didn’t make the grade in actual operation off the pedal. Carpet ,bent linkages, throttle cable issues etc.

[400 Lemans]

You said that you have the correct 69 Quadrajet on there. There is alot of info on how the factory limited the secondary throttle opening on the early Firebird carbs, so they wouldn't outrun the GTO's. Just a thought. Maybe worth a check.