need to know rims sizes!?!?!?!?

I would like to know the rims sizes for a 86 Iroc Rear.. Like I know the bolt pattern is 5X4.75 but what is the offset and backspacing? Is offset considered backspacing? Help me out looking to buy some 17x11's and dont know the backspacing that I need. The tires are 12.4 wide. 315/35/17 Nitto 555R's.. any help would be great, I am pretty sure that with the 97 rear end under there with the extra 2" backspacing I should be fine.... Can someone shoot me some numbers?

This site has all the info you need,

http://skulte.com/adapterfaq.html

Ask the question over on the LT1 or LS1 boards. You have a 4th gen rear, so you need 4th gen wheels...