As you can see here SVRA doesn't even list a Pontiac Engine due mainly to no one willing to try and build one.
http://www.svra.com/wp-content/uploa...c_Firebird.pdf
But they generally require Iron heads.
HSR doesn't even list a specific rules set for a Firebird but look at the Mustang and it'll mimic it.
http://hsrrace.com/wp-content/upload...70_-HSR_16.pdf
I run in Gr-5 in the S1 class
http://hsrrace.com/wp-content/upload...ses-5.8.16.pdf
If I built a 303 I could run in the HTA class, but building a 400-440 is so much easier. I run 5+ events a season so I have to stay with what'll stay together or is easily replaced.
I'd suggest figuring out what Sanction you plan to run in and build accordingly, unless you're just building a track day car.