It's a 72, so unless it has been "trifled with" I'm assuming that it should deal just fine with 87 octane, being a low compression motor.
If the points plate is moving "a lot" then it could be causing your timing to jump around (do you see the timing mark moving around?). Also, you didn't mention age but sometimes the elastomer in the harmonic balancer and age, dry out, and allow the outer ring to slip. When it does, all bets are off as far as timing marks go (not to mention rendering the balancer no longer capable of "balancing" correctly.) If you're not completely sure, then if it were mine I'd start by making sure that when the timing marks say "TDC" that they're telling the truth. For that you need a piston stop that you can insert into a spark plug hole (they can be bought or made - I made mine) and the procedure to follow. You should be able to find it all over the 'net if you look. On a Pontiac though you're going to want to remove the drivers side rocker cover (valve cover) and loosen both rockers on the intake and exhaust valves on #1 cylnder enough to remove the pusrods. Otherwise the valves are likely to hit the piston stop as you move the crank, and something's gotta give when that happens.
If you do find that your balancer is off, I recommend replacing it - AND double checking the new one. I've seen brand new SFI-rated balancers be "off" as much as 5-6 degrees. It's easy enough to fix just by putting on a timing tape, but still you've got to be sure you know where TDC is.