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.

The intake will be the fun part but I think I can pull it off. 301 block with welded in lifter area for strength. I'm making caps for it that are the correct size for an early forged crank, and making saddles for the block. On mine it will have KRE Dports, I don't know if that is allowed for you. I like the short deck to keep the reciprocating weight down. So far I have a solid rod in the cam tunnel and mains so the welding will have minimal effect on the block staying straight. I'm in the process of making 16 parts to put in the lifter bores so the stay round. Right now the crank I have is a 58 370 forging, but it can be offset ground to shorten the stroke if needed.Stock stroke is 3.5625 and the mains are 2.62, both of which have room to change depending on bearings available.

Depends on what sanction and class you run it. SVRA has the old HTA class 303 limit which I think is actually closer to 310 with allowed overbore. HSR has various classes from the HTA 303 up to the S1 class which is 428 for a 69 or earlier.
Our group, Outlaw Vintage Racing, is regional only to the Southeast, we have no cu in limits, only a pwr-wt ratio.
I'd prefer a 303 short deck build for my next engine but it's damn near impossible. A Crank is hard to come by, aftermarket block bore is too big, a 301 takes a lot of extra work and no intake exists for a short deck. If you get one figured out please share, maybe we could pool resources.
Most lead pack cars running HTA are SBC or SBF and run dry sump, and they spin around 8000 rpm. Building a 303 Pontiac out of a 301 to compete against that? I'd like to see it, I just can't figure out how to do it.

I'm working on a heavily fortified 301 block with a forged 370 crank, and was wondering what cubes you're allowed to run. Aftermarket heads ok? I like the short deck aspect, especially for the road race venue.