Wow, you may want to do some more checking before pulling the trigger on this.
I just got a set of 18x10.5 and 17x9.5 modified Z06 style wheels for my 3rd gen TA (’87), and with Z06 tire sizes (295/35/18 R, 265/40/17 F) they look like they’ll fit well (like as in I test fitted stuff with just the wheel and just the tire, I’ll have it bolted on probably over the weekend) with 2” spacers front and rear, maybe slightly thinner in the rear, but it’s a huge rim, there isn’t that much room to move stuff in on the inside of the rear.
Now that is with the stock offsets, 58mm in the rear, which with a 2” spacer ends up being about 7mm positive offset in the rear and the front 54mm about 3mm offset in the front. If you actually want tires tucked in on 3rd gen f-bodies, something around 0 offset works well on the front, and 10-20mm on the rear (the stock 16” rims used a 0 and 16mm offset).
Most of the stock G-body rims are 0 offset, which stick out a little if you put them on the rear of an f-body, and when you guys grab GTA/TA mesh rims for your cars you use just the front, 0 offset rims, and for aftermarket rims it seems that most recommend something between 0 and -25mm offsets.
What I’m getting at is based on that (I haven’t tried them specifically), you’re probably looking for 2.25” – 3” bolt on spacers to make those rims work on your car. I’d be surprised if 2” or anything smaller spacers will work without minitubbing the car.