I got mine from Inland Empire Driveline. Go to there website, I think it has directions on how to measure. If it doesn't, call them they'll help you out.
http://www.iedls.com/
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347, 279/284 .493/.510, TFS TW, Stealth, 750, Flowtech hdrs, Dr. Gas X, 2 Chmbr Flowmaster, MSD 6AL-Dist.& SS coil, Art Carr C-4, Currie 9" w/3.55, alum Drv Shft, susp mods, 4 wheel power disc
In a 66 coupe