I had the same question last November when I replaced the clutch on my '57 3600. The 10" was standard, but the 11" was an option on the '55-'59 3600's. Turns out I had the 11" clutch, but had purchased the 10". My local O'Reilly had the 11" in stock, so I got lucky.
I would buy both so that you have them both on hand and return the one not used.
The 11" does not allow much wiggle room to get in or out, especially if you are doing it on your back. Be patient.
Good luck,
Matt
