On the Jolly Page for "known good part numbers" it says for 1966: AC Delco AC4 (of course any equivalent is OK). 3/4 reach.
http://www.oreillyauto.com/site/c/se...kup&vi=1438134
16 to choose from, take your pick. Be careful of what they gave you, an Autolite #66 does not seem to cross to 1966 GMC 305. It is, as you say, a 3/4 reach, copper core. If you are saying that you measured the reach on what they gave you and it does not crush gasket, it has to be a dirty hole/threads or somehow the base diameter is too big? Make sure you find out what the torque should be and torque the correct plug to spec. Go to the counter at O'Reilly's and measure length and dia. of one of the one's I linked and compare to a #66.