Spark Plugs for Slant Six?

Headed out to a small weekly parking lot meet this AM. Shifted into overdrive @40 mph and experienced what felt like a U-joint failure! Turned around and slowly headed home, no vibesas I worked gently through the gears. When lugging the engine I again got the vibe, like a major spark miss. Got home, checked the wires, #2 plug wire loose on the plug, snugged it, same issue when short shifting.
Hmm...vacuun leak? Nope dizzy & PCV hoses intact, not leaking.
Note: Car has a'66 motor, and I've only put about 200 miles on it. Dizzy is stock vacuum advance, has the pertronix points eleminator in it.
Carb was recently rebuilt, Used the thick base gasket, nuts took about 3/4 turn to snug. Bingo problem solved.
Now how this relates to this thread, checking the plugs I found NGK GR4's, and they still have the washers!
All six plugs are a nice tan color. no wear on the electrodes, gonna pull the washers and get new drool tube gaskets.
Everyplace I look this is this recommended plug, I understand it is 1 heat range higher, but the tip color is not indicating a lean condition. No oil burning or fouling.
Curious if anyone has had an issue running these.