Misfire at cruise, popping/afterfire at 3000RPM

Looks like a case of Holleyitis. There is a vaccine, it is called Carter.....

Some cyls maybe misfiring because of the fouled plugs, which would account for the difference in header temps.
Plugs should be heat range 5 in NGK, no colder.
I would remove the centre carb & make sure you have 0.040" or less of T slot showing. More than 080 would probably account for the rich idle & indicate bypass air is needed.