I think I found the problem.
Gut instinct was to look at ignition.
So, got to looking at the HEI, pulled the coil & cap, and the center button was in a couple of chalky pieces.
You could see a trail of powder running in 2 opposing directions.
How that thing ran so well coming home is beyond me.
I'm guessing that the button broke, and as it disintegrated there was a bit of crossfiring that went through the exhaust instead of the intake.
Side note: someone before me put an Accel Super Coil on the top, but the special screws that attach it to the cap have a black oxide coating that was kinda rusty.
The "while I'm there" means new cap, coil, cover, wires, air filter, and breather filter (a GM thing)...