The original plug used was a Champion D16 18mm with 1/2 in. reach 7/8 hex size and was a non-resistor type plug.
The AC direct replacement shows a # C86 but I don't have the specs on that plug.
Also compares to a Autolite #386 with 1/2 in. reach 7/8 hex size but it is a resistor plug.
Best bet is to go the NGK AB6 with 3/8 in. reach, 13/16 hex and is non-resister.