When you check and adjust timing make sure your idle speed is below 800rpm or you risk your mechanic advance interfering and giving an incorrect reading. Here's a link to the light I use, it works with multi spark MSD and all other ignitions, some rental stores rent this light.

If you have adjusted the 600 Edelbrock correctly something is wrong with it, it should be the same or slightly greater vacuum than the 750 Holley, a drop from 19" to 12" is wrong. The gasket I referred to is not the base to manifold gasket, but rather the carb body to throttle plate gasket, make sure it is correct size and not allow vacuum to escape. I always check the gaskets in a kit to be sure they are correct, you'd be amazed how many are not or have multiple gaskets to satisfy other carbs as well.

The popping through the exhaust and intake both is concerning to me, especially the loud pops you describe. A manifold/carb vacuum leak can cause intake pops because of the lean mix (concerning in itself), but along with exhaust pops of magnitude sounds like something else. I've experienced this before with a bad EGR valve and smog pump system, but I'm guessing you no longer have that installed. Thus the burned valves or timing chain thought. However, the Holley 750 running good kinda rules this cenario out.

So back to the Edelbrock, a 7' vacuum drop is significant enough to really muck things up, but this usually causes a lean condition (intake pop yes), the exhaust pop is usually a late fire issue or an exhaust manifold leak pulling in enough air to ignite any left over fuel in the exhaust (usually rich mixture contributes to this). But it does seem to be carb specific, possibly if the vacuum leak is effecting the metering circuit, which with that much of vacuum drop it most certainly is, and you are alternating between lean to rich as the metering circuit reacts. I believe we just found your culprit.