Rkeohane- I would avoid Rt. 6 in Eastham, Wellfleet and up if you can. I ride down there every summer and 6 is unrideable from the Orleans rotary to N. Eastham. Even in Wellfleet and Truro where there are shoulders, the cars are averaging 60mph on rt. 6.
Attached is a route that I ride in those towns.
http://www.mapmyride.com/ride/united-st ... 8052105663
I don't know about the rest of your ride, but the Boston to Cape Cod Bikeway route would be a good starting point.
http://www.massbike.org/bikeways/ccbw/
Good luck,
Neil