The correct pinion angle depends on the angle of your trans. You'll need an angle finder (available from Sears for about $10) to measure the angle of the rear face of the trans. Once you have that just reverse the angle and you'll have the proper pinion angle. The optimum is for the angles of the trans and pinion to be parallel, however a difference up to 3* will work.
Here's a graphic that shows how pinion angle can effect the operation of your "U" joints.