Small Block & Big Block Chevy Specific - One Glowing Primary..help!!

i hope this is the right spot to put this post...ive done searches and tried a bunch of things...

I have a 355 sbc in my race car with a carburetor...i have been racing it all season with NO problems and been running stronger every time out.
I recently developed a glowing primary on the passenger side #5 cylinder i believe?? i have checked all intake manifold and header bolts, replaced plugs and wires, and it is still backfiring and glowing just a little.
it ran fine at the track and drove fine until i was on my way home yesterday. it feels strong as always but just backfires a little. i dont think it is a bent valve and i checked the rockers and they seemed right on.

ANY SUGGESTIONS????? thanks!!!!!!!!!!

im going to do a compression test to check the valves...and then i guess i'll make sure there is no vacuum leak...any other suggestions?
if it were a spark plug wire it would barely run

these symptoms sould like a lean engine condition "car runs stronger every time out" "glowing primary" but it could also be head gasket with only one tube glowing. As far as bent valves that is easy to check with a compression test so eliminate it first. Next pressurize the radiator and see if it loses any, this won't eliminate potential problems but it may find one.

good luck.