Don't just set the angle at the rear end. You have to consider the angle at both ends of the driveshaft to do the job correctly. Here is one of the best explanations of the procedure I've seen.
http://www.rosslertrans.com/Pinion%20Angle.htm
You can get an angle finder at Harbor freight pretty cheaply that works good.
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68 GTO,3860#
Stock Original 400/M-20 Muncie,3.55’s
13.86 @ 100
Old combo:
462 10.75 CR,,SD 330CFM Round Port E's,Old Faithful cam,Jim Hand Continental,3.42's.
1968 Pontiac GTO : 11.114 @ 120.130 MPH
New combo:
517 MR-1,10.8 CR,SD 350CFM E's,QFT 950/Northwind,246/252 HR,9.5” 4000 stall,3.42's
636HP/654TQ
1.452 10.603 @ 125.09
http://www.dragtimes.com/Pontiac-GTO...lip-31594.html