All Pontiacs up to and including 68 used the 8 bolt. Then, all 69-79 went to the 11 bolt pump. In addition, there are two different 11 bolt water pumps. They are referred to as the short (early 69) or long (later 69) water pump. The one you need depends on the pulleys that you have. If you try to use the wrong one, your pulleys will no be lined up. The good news is, if you need the longer one, you can use the short one, you will just need 1/4" spacer that goes behind/inside the waterpump/fan pulley. Whereas if you need the short one and buy the long one, you are screwed...

If I remember correctly one is 3-1/4" vs. 3-1/2". If you are unsure which one you need, always go with the "later" one which is the 3-1/4" because as I stated above, you buy the spacer from almost any parts place (the look like huge washers with all the holes drilled in them for the fan bolts to go through. Most of the times they come in a package of several thicknesses so you can create the thickness you need.

Now here is a trick if you need to use a spacer inside the fan pulley. Put a dab of silicone between the spacer (or spacers if you need more than one) and let it set up before tring to insall because it can be difficult to line up all the holes in the spacers, pulley, fan etc. Just don't use too much as you don't want to creat a "wobble" in the pulley etc.