They make a tool to do that, but it's easy enough to build your own. Someone posted his version here I think, some time ago.
http://www.jegs.com/i/Percy's/760/01201/10002/-1?CAWELAID=1710534765&CAGPSPN=pla&catargetid=230006180003463243&cadevice=t&gclid=CJ2-s9PugcoCFUeUfgodH3AKewThat said, when I went to 7x15 rims on the bird, I did my own measurements. Then a friend with a 57 bird (and the same salt flats wheel) told me the local Les Schwab would be about the same price for the wheels, and would guarantee the fit. So, I went over and talked to them, and they confirmed it. They did a bunch of measuring, and came up with the same backspacing I did. 3 1/2" front, 4" rear, IIRC.
I told them I was ordering Diamond Back tires, no problem, just bring them over.
They fit wheels (they needed a little ground relief to clear the front disc), mounted the tires for the standard charge, replaced a front stud, no charge, and sent me on my way. With 225/60's in front, and 235/60's in back, it's real tight. On a hard left turn, I got a little rubbing in back. Turned out the body was 1/2" off center. Centered the body, and all is well. Until he sent me over, I would have never thought of using a chain tire store, but it worked out pretty well.
Hopefully, someone with a CV will chime in with more measurements.
miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ