If you made the decision to go with the actual 1' x 3' lumber, now is where you will use (if you opted to with regard to the U-shaped frame, it will come into play really your lines are laid out). Screw a short lived 1' x 3' support rail on the wall, aligning the top edge of this rail but now line for that bottom fringe of the wall cabinets. Attach it by driving couple of two inch screws with rail