Why are my distributor cap terminals burnt?

Assume you're talking about the outboards carbs on the 6-pak.
That's good. Now you've isolated a problem. Leave them disconnected for now. It will work fine on two barrels, certainly for street use.
Yes, sorry. I’m talking about the outboards on the six pack.

I only blocked the rear, running on the center carb and front outboard. So essentially running a 4-barrel, assuming the front is opening.

I agree, I’ve isolated a problem, perhaps not necessarily THE problem. I suspect there are multiple things going on here, and next up is the distributor.

I wish I could better describe what is happening when it hits higher RPM. There’s a guy not far from me who works on older cars, maybe I’ll pay him a visit. Simply because it might be easier for an experienced ear to identify the sound as opposed to me trying to describe it.