In trying to find the thread I was thinking of, I came across this thread on early bracket prototyping and whatnot for the LS1 brakes. There's a few inner wheel barrel diameters thrown around in the first page or so, haven't found specs for GTA wheels though (or my own wheels, to compare to). The few inner diameters I have managed to find measure at roughly:
-15 1/4" for 17" Firehawk Wheels (Posts 48, 49, 52)... post 53 possibly as well, where the Firehawk wheel guy mentions getting 14 3/4" on the very inner diameter, by the mounting flange... doesn't say what wheel, but assumedly a 17" Firehawk
-15 3/8" for a "4thgen wheel" (doesn't say if it's a 16 or 17; Post 51)
-15 1/2" for a 16" IROC (Post 45)

Most of the basic measurements are there in the first post of the thread I linked, but it doesn't include the dimensions of the actual calipers... might later on, there ARE 17 pages to go through as far as rotors though, both 1LE and LS1 measure at 12" in diameter, but the 1LE rotors are thinner (1.05") than the LS1 rotors (1.25"). LS1 pads have a couple square inches more pad surface as well, so I'd be willing to bet that the calipers themselves are going to be a tiny bit bigger with the LS1 setup.
The IROCs reportedly clear LS1 brakes with no issue, which would make sense with the rotor diameter being the same as 1LE. I'll see if I can dig further in and find some info on Firebird wheels...

EDIT: Post #174, looks like the LS1 brakes rub up against the mesh of the wheel, but looks like they clear the barrel.... might just need a small spacer to get em to fit right. The first post in the linked thread mentions the spacer as well, with GM part number 14091902 (0.030")
Post #279 has pics of the clearance with GTA wheels, Ed from Flynbye says he needed a 3/16" spacer for them to clear the face

Do also keep in mind that this thread is from '03, and everyone was making their own brackets at this point. Wheel clearance very well may be different with the BigBrakeUpgrade brackets, but at least barrel diameter doesn't appear to be an issue

 

I think the issue is when you use sticky weights (which everyone uses now). The spacer issue probably isn't an issue with an actual C6 hub because the hub will stick out farther. I don't like the 3/16 spacer on factory hubs with LS1 rotors because you don't have any material supporting the wheel (hub centric). You can see that in Ed's photo:



The C6 hub sticks way out, so that wouldn't be an issue. So Maybe use a 3/16" spacer and be aware of wheel weight placement.

 

Anesthes, I’ve come up thinking of something. Use the LT1 rotor as you’re not looking for a large diameter rotor and if it’s possible, try out the Wilwood D154 calipers. They’re aluminum so that’s an immediate upgrade from early 4th gen brakes, they’re a direct bolt-on to the third gen spindle and intended for a 10.5” rotor which is what the early 4th gen’s had, but even though you’re using the Hawks spindle maybe there’d be an easy adaptation you could make to mount them. I don’t know, I’m just spit-balling here. Those Wilwoods come in both single and dual piston too. The nice part is you could work it out with the factory junk before committing to buy the calipers to see if there’s any possibility of it working.

 

That's an interesting idea. I'm running the d154 on the back of my model T (hot rod). They just use two 3/8 pins to retain the caliper. Are they acceptable for front brakes?

I'm gonna have to research this more. Thanks for the idea!

 

The D154 metric calipers are the base factory front caliper for these cars and many other GM cars from the late 70’s up into the 90’s.

 

I'm talking about aluminum d154s. When used on the rear they are mounted with just the guide pins

I wonder if the aluminum ones can be used kike that in the front or if they need the integrated pad stops like factory.

I wonder if it would bend the pins.

Hmm.