JG, both DHMag and baddbob suggested changing to a colder plug. Toddle on down to Autozone and get the colder plugs and gap them properly. If using a traditional distributor with separate coil, gap at 0.035". If HEI, gap at 0.045". Pick up one of those little plug gap thingies they keep by the cash register. Looks like a large coin and has a hole to affix to your key chain. Do that and see what happens. Report back with your findings.
Yes, it is possible to retard the ignition.
The problem is that unless you've verified TDC on the outer ring of the harmonic damper where it lines up with the pointer on the front cover, then you are flying blind. Oftentimes, the ring slips on the elastomeric material that is placed between the damper outer ring and the inner hub. If that has happened and you haven't verified the true TDC, then any further attempts to time the motor with a light will prove futile.