If I recall, Jim Hand talks about the X crossover for it's ability to calm down the exhaust noise while maintaining performance. In the later parts of his book, he talks about the trade offs between exhaust configuration, noise and 1/4 mile times. I think the X crossover was a winner except for the complexity. And he was not discussing the Firebird where fit might be more difficult.

I would be interested in hearing about the concerns with Pypes system. If not their system, what will mail order and bolt on in the garage without hauling the project off to a shop on a roll-back?