http://www.6066gmcguy.org/partnumbers.htm list the plug as an AC Delco AC4. AC Delco catalog says 14mm.

Putting 1966 GMC 1000 305V6 into www.napaonline.com gets a Champion CHA 405 plug with 14mm threads.

I have access to a couple of these motors but am not where I can get to one right now to give you anything more definitive than this googling.

Edit: One more thing, you probably will get more information on this motor in the 60-66 subforum than in this engine section here at Stovebolt.com, just my take on it.

Last edited by R-Bo; 10/03/2011 6:03 PM.
R-Bo

1959 Apache 1/2 ton Big Window, Short bed Fleetside (under reconstruction)
1966 GMC 305V6 in the family