READING 70 SWINGER 340 SPARK PLUGS

What is 'a run'? If you wind it out in all 3 gears, then let off and cruise home, and only then pull the plugs, you won't see what the fuel conditions are at WOT; the engine operation at cruise going home will mask it. If you want to take plug read for WOT, put in a clean set, drive to where you can got WOT as far as you can, then at the end of the WOT run, cut the ignition, drift to a stop and pull plugs and read. (Of course, you gotta be careful with losing any power steering and maybe power brakes, and the old steering columns that locked when you cut the ignition!)

What fuel is being used? The modern pump fuel seem (to me) to look lean on the plugs.

The look of carbon on the outer edges looks rich. And I would guess that it is rich at cruise and probably so at idle. Can't say myself for WOT. 13.5 AFR is richer than I run on flat level ground, steady throttle, 55-65 mph; I look for around 15 under those conditions.

And beware that AFR's will read lean at idle until the engine is fully warmed up. So don't take AFR readings while the engine is cold or ever warming up.

Can you tell us a bit more about the engine?