1988 Formula 311 420's, B/W's, TRS

I was changing the plugs today and noticed 1 out of the 16 was different. The book says they take AC Delco MR 43T. There were 15- .MR43T (notice the period) and 1- MR43T. The difference is in the length of the shank of the plug. The oddball was about 3/8" longer.

My question is what plugs should I run? I just picked up the boat in the fall and it was previously marina maintained ( not that means anything). The longer one isn't hitting the piston or anything so performance wise which one is the better choice?
Is the idea to get the spark closer to the piston?

Thanks
Chris

I'd go to the Tech section with this one; It's a Mercury question, not really a Formula question.

Yea I have it there too but last time I posted there I never got an answer, and I know alot of people here have a similar setup.

I use MR43T.