It seems that V8 Pontiac engines up to 1976 used a 2.75" crank register for the flywheels to mate to.
Information seems to indicate that in 1977, a switch was made to 2.5" (actually might be 2.49" crank register size. Some say "1977 to 1979" was the timeframe for the 2.5" cranks/flywheels.
If your car is known to be stock, I'd say it's pretty likely to have the 2.75" size. But if it's been rebuilt, or you don't know the complete history, it is possible a late model crank could have put into an earlier block...
Another thing is if you do have a small register crank (the 77-79 one), and you want to use a larger/cheaper large bore flywheel, there are allegedly spacers made by somebody to make up the size difference.