I need some help guys

[Zefhix]

We're running the corvette Y2K rims on our 70 Chevelle but I'm not sure how to calculate the size adapter/spacer we need for the front wheels without physically having the car here. The rim is a OEM spec 17x8.5 up front and we'll be running a 255/40 series. I have to order the spacers now, anyone have a thoughts on what a safe bet would be? Is 2" too much?
I appreciate the help. I can't seem to find detailed specs on the backspacing of the Vette rim so I figured I'd turn to the expert egg hunters here. Thanks in advance!