Individual header tube temp. Egt. Varies

Hmmm to my memory ..when I shot my header tubes years and years ago on a fairly warmed up 340 they were around 500- 600. Lean idle.
You guys run who run out of the box intake manifolds need to realize that none of the runners flow the same ,none of them distribute fuel the same.

Dual planes, center ports...lean.
Just truly get your idle dialed perfect, dial your carburetor in ...or as close as you can get it... then go check those Center plugs...they will be the cleanest or even white