I am looking at getting a set of 17x9.5 ZO6 aftermarket wheels.

It states an offset of 54mm.

What size adapters will I need to put these on my 88 IROC?

Anyone?

I did a search and all I could find was, " Most people use a 2" spacer, but some use a 2 1/2".

I contacted Jegs who sells the wheels in question, and they said the wheel has a 7" backspacing. Which would need a 1 1/2" spacer to attain the desired 5 1/2" backspacing.

Does this sound right?

I wrote a tech article, linked off the main page here at TGO:
http://www.skulte.com/adapterfaq.html

I'd recommend going with 2" or 2.25" adapters on all four corners. You could go thinner in the rear, but then the rims will tuck in more than the front, and most folks liked to have them even. They shouldn't stick out at all with 2.25", and a 54mm offset, and would give you plenty of clearance against the strut in the front. It seems GM's tolerances make rims sit differently in different third-gens, so sometimes it ends up being a trial and error thing. Also, I've got no problem exchanging them for a different size, if you're not happy with the final fit.

Andris