Spark Plug Advice Needed

Dan,
That book is David Vizard's How to Tune Holleys, CarTech SA216.
Ign timing in a Carb book? And ign timing mentioned several times?

That is because DV gets the relationship between idle timing & carb idle setting. Many do not get it......

Idle timing is done first before touching the carb. Quite often, problems such as off idle flat spots will be cured or reduced once the timing at idle is correct. The usual problem is not enough timing at idle, not too much.
For reasons known only to Chrys, they did not use man vac adv [ MVA ], instead used the useless PVA. GM used MVA. My 1966 showroom stock Pontiac idled @ 26*: 6* init + 20* MVA; with a very mild cam & 10.75:1. [ Chevs used about 24* ] With more cam & lower CR, vac is reduced & even more timing is needed.
'Lots of carbs are rich at idle'. Only if there is a manufacturing fault where the mixture screws do not seat against their seats.....& that would be rare. If the T slot position is correct, screwing in the mixture screws will stall or almost kill the engine from lack of fuel.
The idle fuel feed is sized to provide enough fuel for transition [ to the main cct ] NOT for idle which requires less fuel & is regulated by the mixture screws.