I used a Holley 95-20 bracket, same as the two square hole bracket you described. Instead of the cable I used the rod style linkage typically used on Chevy's. I cut the top square hole off the bracket just to clean it up. Trouble is, now that Dynamat, Dynaliner and carpet are installed I don't believe I'm achieving full throttle. There is little latitude to gain more travel with the rod set-up. If I can not solve, I was thinking of exploring the cable option.

I don't know if there is bracket commercially available for the original cable. It's been a long time but I believe the original Firebird cable uses a round hole instead of a square. It may be simple enough to cut the upper square hole off the 95-20 bracket and weld on the end from a round hole bracket in it's place?