360 running on 4 cylinders

Spent this rainy Sunday troubleshooting some more.

• Changed the coil, no change
• Changed the cap and rotor, no change
• Changed the plugs, no change
• Switched back to the Carter, no change (it does run smoother with the Carter probably because it's not running rich and fouling the plugs like the Holley was)
• Removed the ECU and sanded all the paint off the mounts and fender wall. Reinstalled with dielectric grease, no change.

I also used my timing light to check all the plugs for spark. All seemed to be good steady flashes except for #6 it was flashing really intermittently or multiple times per stroke. Think it could be the ECU?

Appreciate everyone who has made suggestions. I still need to get a leak-down tester and prove each cylinder.