Getting pulleys aligned properly on a Pontiac is the stuff of nightmares. For some odd reason there were a significant number of factory variations from Pontiac - why, I have no idea. Example: In 1969 alone there were three different water pump pulleys, all with different back-spacing, just for A-body (Tempest, LeMans, GTO) cars.
Your starting point should be making dead-nuts positive that all of your accessory mounting brackets and pulleys are from both the same model year AND the same body style - and that's just to start. If you've added any aftermarket parts into the mix, then get ready for a bumpy ride.