I have a friend of mine that has about the same year firebird and it has a pretty stout running 383 roller motor in it with a motor plate set up and midplate, believe me everything on the car was a mjor pain getting the motor in there granted his is a dragcar and clearance wasnt as big an issue as what yours will be but still its was a major pain.
As far the fule tank he had to cut out part of the tank and have a sump welded in and it works nicely, sure makes for a sano install, there's not much room in the back for a fuel cell unless you wanted to gut it which he didnt and I'm sure your not either, another problem was the weak stock rearend which crumbled almost immeidatly, but now he has the curry bolt in 9 inch unit which is almost bullet proof but it wasn't no easy install either. Goodluck, Charles