After thinking a little bit, if the engine was running lean on the primaries AND the timing was a little too advanced - at part throttle and low load situations, you could have been running the engine lean - not detonating because of low load, but still running way hot. This could have warped the head over time and caused the gasket to expire. This could have been happening before or after the new carb.
In fact, if it happened with the old one, and then you put a more efficient carb on it, you could have been making much more power than the old lean carb, and you blew the weakened head gasket once you put the new carb on.
The reason it didn't ping under heavy acceleration, though, could be because the secondaries richened it up enough that it wasn't in the danger zone while you were hard on it, just at part throttle.
Its a wild guess, but when you pull it apart, check the head straightness.
Too much initial timing with a slow (read: MSD stock) advance curve could also make it ping at low revs, but not at higher.
I think it put more power into it than it had been seeing and possibly ran the timing too advanced, and the head had warped over time, allowing the extra power to kill the gasket.
<font size=-1>[ This Message was edited by: thekingofazle on 4/14/06 1:27pm ]</font>