Is yours up top or down low?
I used to work on these at dealership when new, but has been awhile....
You will have to hunt the web, I only found 2 yeah tubes ... Not the best but you can see what they did.
You are in the age range where GM was switching over to single serpentine belt system. Prior to that the air pump was a mounted usually at the lower front on the passenger side. The bottom bracket was usually crescent shaped and was where the adjustment was made.
The pump usually had its own belt typically 3/8 wide. This was also a short belt that just went from the crank pulley to the pump pulley. To remove the pump you need to remove the pulley as the bolt heads are usually behind the pulley and can't be removed until the pulley is removed.
The top bolt was usually a through bolt that mounted the pump to the block and this is the bolt it pivots on. The lower bolt was where the adjustment was made. There also might be a short bolt at the lower back of the unit, and loosened to get the unit to move. You tighten it back up once the adjustment is made.
Can you post a pic of what you have?