Start with the basics. Remove no. 1 plug, bring the piston to TDC and check that the balancer timing mark mates with the mark on the timing cover (two piece balancers have a habit of spining on the hub). Then make sure that the rotor is pointing to no.1 on the cap (remember that no. 6 is 180* across the firing order and at TDC at the same time - the only difference is the camshaft position and compression stroke). If the distributor hold down was loose, the distributor may have jumped a tooth or two.
If everything checks out, turn the engine over by hand with the dist. cap off - first in a clockwise direction, then in a counter-clockwise rotation to check if you have a lot of play in the timming chain. Lots of crank rotation with no distributor movement means timming chain problems - not unheard of in a Pontiac 400.
Good Luck, keep us posted.