You may need to find a new shop, with a mechanic who has worked on first gen Firebirds, as well as an installation aid (big hammer) or a properly fitting set of headers.
If your headers say "fits 1967 to 1981 Firebirds" you can be quite certain that they will not. The first and second gen are NOT the same.
The drivetrain on first gen Firebirds with Pontiac engines and transmissions is offset from center. They were that way the day they were built. As long as you have the correct motor and trans mounts installed and they are bolted down properly, you should not have an issue.
You will see the offset all the way back to the differential...see the driveshaft alignment.
