You could also go with the plywood sub-base and biscuit join (or spline
join) the board together. That way you could simple run three rows of
screws underneath. One row right up the middle with no slot required
since there will be no movement there. And the other two rows on the
edges runinng down the length. Naturally leave room with a slot for
expansion. If you aren't sure play it on the safe side and go more than
you need. When you are done, monitor the wood movement over a year
(each season) to see how the wood moves. Then you'll know for next
time.

Woodwork Safely,
Jim Barry
http://www.woodchuckcanuck.com