The 3rd gen f-bodies would be a good match for the fox bodied mustangs. The performance differences between the two should be able to be balanced with weight . IIRC, the Firebirds were rated 5 or so HP less due to the intake configuration. IIRC, the later 5 liter sefi engines were rated at 225 hp. They revved easier and higher than the 5 liter f-bodies. The Fox cars were apporx 200 lbs lighter to start with, had lesser brakes(10.25 or so front, and drum rear). All the later F-body rear discs were the same as the 1LE cars had. The f body suspension is better, but the Mustang felt pretty nimble due to it's size/lower weight. I don't know the SN95 cars that well, but IIRC, they lost 10 hp or so for some reason. I'd guess they are heavier than the fox cars.

Both cars have a HUGE aftermarket support system with exhaust stuff, pulleys, gears(more so mustang) and more.

The Automatic LB9's(305 TPI) had a different cam than the 5 spd cars did, hence the lower HP rating. 89+ 5 spd LB9's only came with the good cam if a 3.42 or 3.45 rear gear was specified.

If the LB9's are deemed too strong for the class, the only other option would be the 5.0 HO 4 bbl cars. Terrible electronic 4 bbl carb. It had fuel starvation issues, and no matter how many people told me they knew what they were doing, I never found anyone who could make one run right. These cars were only available for a couple of years, as they disco'd them as GM got production ramped up on the TPI cars. IIRC, they were rated at about 190 hp. I'm afraid you would have to get them pretty light(relatively) to make them competitive with the 10 year newer SN95's. The 170 hp cars might make a mid pack ITS car if you could get it light enough.