How thick of a wheel spacer

I have a rearend that has been cut down 2.5 on each side. I was going to mini tub and move the shocks but im thinking about staying stock suspension. How thick of a wheel spacer would I need to use with a 15-10-6.5 back space?
Thanks

What kind of car?  

Its an 89 mustang hatch.
Thanks

The rearend is a moser M9 40 spline gun drills. Im looking toward leaving the car ss and go with some qa1 DA coil overs in the front and some qa1 stockers in the rear and run a msd programmable al2.

I'm just guessing, 2.5"? It seems that a 10" wheel with 6.5" back space is what works with a stock width, so go with a 4" back space wheel. I have 12.5x28 ET Streets on 6.5" backspace wheels on my Fairmont with 1" off each end and they barely clear the pinch weld. I mini tubbed the car so there's plenty of room on the inside.

Using wheel spacers with 11/16 x 5/8 drive studs is usually ok as long as the spacer isn't too thick. But using spacers with the smaller regular 1/2" wheel studs isn't the best idea.