Hate to see that happen to such well put together car – the blow-off valves are a must with that intake – check your carbs now as the explosion usually takes out the part of the casting where the air mixture screws seat into – especially if they are edelbrocks as they are more prone to the problem than the carters.

The big bang probably started with the #7 plug being it was fouled – possibly your lash may have been slightly off or that valve was hanging a little being the engine was cold. It does not take much to set them off.