The 4x 12 top sits on the inside of the left and right sides (duh!) and secured by 3/8-16 All Thread. Of course ACE hardware only carries it in 36" and 60" lengths. Outside to outside, the bench is 37" wide. Had I known, I could've cut the 4x12 to 35" and used 36 inchers. Yet another reason to get the hardware first, then settle on final dimesnions.
The black round things in the right photo below are from Lee Valley - 3/8 - 16 Tension Rod Nuts, part number 05G0710. They look a lot nicer than a nut and washer.
In keeping with my over design tendency, and having options is nice, I over did it with the pan head screws on the back. A dozen on each side of the bottom "stretcher" and ONLY four on each side of the top stretcher, along with the ones on the other side of the stretchers should make the back rigid - and still have the option of extending this bench later if need be.
