Stiles recommends using lag bolts 1/2 inch thick and at least 6 inches long.  We used lag  bolts that were 1/2 X 8 inches long.       
 

We drilled pilot holes 3/8 inch in diameter, then attached the  steel brackets using  the 1/2 X 8 inch lag bolts, with the 2 X 6 foundation boards already in place.