How to remove spark plugs from 1966 Plymouth Valiant

Ok ... APPARENTLY, the heads are from a later model requiring 5/8 hex plugs BUT the plugs that are in the engine now are 13/16 and they're not even screwed in tightly just TOTALLY wrong size! My VERY patient husband is painstakingly removing the old plugs with vice grips ... Aaarrrrgggghhhhh!!