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50/50 ATF and acetone mix left to soak for a few days.. a socket with the end ground off, combined with a 1/4 breaker bar to enable me to seriously push the socket on to the remains of the plug thread..
Take the point on piston contact risk.. I've got a bore scope and will check before putting the right plugs in.. it 'should' be alright in theory.. 71 open port rectangular port heads and pistons that allow it to run on premium pump gas - but worth a look!
Looking at the picture of the plugs, if there was any contact with the pistons the electrode end would be smashed shut. I don't see that on any of the ones with the ends shown. You should run a spark plug thread chaser { not a tap } down each hole to clean up the threads of any deposits if the plugs didn't thread all the way in. Buy the correct plugs and you should be good to go.
Probably late to the party, but I'd check the casting number on the heads. One of the pictures above shows a taper seat plug right next to a gasket seat. A taper seat plug in a gasket seat hole (or the other way around) is NOT a good thing. And possibly the root cause of this show.
