[Walt Fricke]

Well, as soon as you get your left side carbs off on that side you will know who the culprit is, and confirm the diagnosis. Taking the intake off with the engine still in the car may make pulling the engine a hair easier, although pulling the intake is easier with the engine up on a stand, which is where it is going anyway.

If it is what all think it is, you will be pulling that head, which means the cam and cam carrier.

While my money (well, guess, prompted by Mike) is that the last guide replacement cracked the boss, and your overrev had nothing to do with it (your springs are good for 8,000 rpm on occasion), with the cam carrier off it isn't much extra work to pull the other two heads off to see if anything is amiss.

With the engine out, doing a precautionary leakdown on the other bank will be easy if you don't find valve marks on the other piston tops.

Let us hope you were lucky, and this spit right out and never got down into the combustion chamber. It doesn't look like that happened, as it isn't beaten up enough. Valve snapping shut at just right time pinballed it up out past all the obstacles in the carb venturi?

The misfire? Alas, that may indicate the intake valve bent during all of this. But you are looking for a new head most likely, so.