Don't do what I did the first time around and center the wheel in the well, this put the tire too far under the car and didn't look right! You need to decide if you want to roll the fender lips at this point. Then figure you want the wheel/tire .5-.75" from the outer fender lip and measure back to the axle, this is your offset measurement and will give you something to work off of.
What I prefer to do in your situation (already have the axle) is to use an old wheel for mock up. I use fender washers to shim the wheel out off the axle untill I like what I see then do the math.
Good luck with it and be sure to post pics!!
