I've tried those plugs on 318 and 360 but not 340 make sure they clear the pistons (they were a plug developed for AMC 6, really smooths out the idle)
there are many versions both 13/16 and 5/8 hex
should work as you do not seem to have the high compression 340
"if they close up the gaps" when you turn it over use standard projected core plugs- use a breaker bar screw the plug in a couple of threads then roll the motor over
tighten the plug a little, repeat if they clear tight open the gap and try it- then run it this plug type works wonders on any low compression motor where it fits
I was with AMC when we made the switch from rn12yc style made a big difference, we started retrofitting with feedback then AMC put out a service bulletin (PS real reason it cuts misfires and therefore smog)
thanks for the heads up on the pistons- hard to tell from the pic and someone thought they were kbs
you may be able to get a leak down tester at an auto parts store rental you will need a compressor
did you try squirting some oil in the cylinders on the low side and repeat the compression test? with the throttle open and ignition dead don't want no fires
