.whichever plug you choose, use this rule of thumb for your gap,..
The greater the amprage the mag makes or out of your msd box,. the less plug gap you want.
Msd mag 44amp,... .013
Msd mag 20, 12, 10amp, ,.. .016-.018
Mallory mag 5, 4, 3amp,...016.020 depending on amps
modified Vertex or anything under 5 amps you would go for .020 and less and so on.
the deal is,. the more amps your mag or spark has, the less distance(gap) it wants to jump,
the less amps you have to make spark, the greater the gap should be to light all the fuel.
SMOLDERY SMOKEY BURNOUTS TO YOU ALL