I've posted on here several times about how to find the correct timing settings for your Pontiac engine. You should be able to find the information about the procedure using the search tool. Short version: Initial timing setting is mostly unimportant. What matters most is the total mechanical timing (vacuum advance disconnected, since when the motor is pulling hard at wide open throttle, it's not making vacuum to activate the vacuum advance). Set your total timing to the value that produces the best engine performance - then use a light to READ (not change) that value at idle so that in the future you'll know how to return to the same setting. (Reaching in to adjust the distributor on the engine with it running at 3000 rpm or so can be a little daunting.) If your distributor is "weird" (i.e. damaged or has been trifled with by someone in the past such that the advance curve is way out of whack) and causes your engine to be very hard to start (i.e. advanced so far that it trys to run backwards during startup) or overheats (because it's extremely retarded) - then you'll want to repair/replace the whole distributor. Otherwise, the 'total' setting is the way to go. A good starting point for most Pontiacs with open chamber iron heads is usually around 34-36 degrees total. Closed chamber (1967 670 and others) and aluminum heads tend to "like" a little more, depending on combustion chamber design.
Example: My 69 with ported aluminum heads seems to like (i.e. make the most power) with 36 degrees total. My distributor (Davis Unified ignition HEI) has 20 degrees of mechanical advance baked into it. That leaves me with an initial timing setting of 16 degrees, which the engine is fine and happy with.