On Thu, 25 Oct 2012 18:30:48 -0400, Dick Snyder wrote:

Thanks. The widest board will be 7" x 74 1/2". I was most worred about
length but I do have room on width as long at I place the two end
pieces, which will be at a 90 degree angle to the long boards, with
screws in oversized holes so the long pieces can move.

Movement along the length is almost non-existent. Your end pieces are
"breadboard ends", lots of info out there on those. Depending on the
finish, 1/4" per 12" may or may not be enough.

Dewaxed shellac or oil based polyurethane do the best job at slowing
water vapor from moving in or out of the wood, but nothing will stop it.

--
When fascism comes to America, it will be wrapped in the flag and
carrying a cross.