No, not '66 only, but, it is crank specific. The early cranks have a smaller register where the flex plate meets. I believe it's '67 and earlier that have the small hole in the flex plate, and '68 and newer have the bigger hole. There's not lot of difference, but, you should match them. I assume you have the early forged crank? I'd have to check mine....it might be the later one....I just can't remember. I'd take $10 plus the ride if we have a match. I'll measure the hole in mine tomorrow and let you know what I have.