The sliding tail vices I've seen blogged (see previous posts) all have re-enforced the front section of thier bench with steel angle. The reason for this is that the force of the vise screw has the un-desired effect of bowing out the front of the bench.
To simplify construction I pre-installed the angle prior to glue up - which is of course easier then trenching out the required space. The top angle is 40x40mm steel angle. 5 x ~40mm x~4mm lag screws to join it.
One of the innovations of my design is placing the parallel bar under the shelf. I re-enforced the bottom front stretcher with two 30x30mm angle. I'm still uncertain of how the forces will play out, but even with me sitting on it, it has minimal flex.
Although, quite comfortable with wood, metal is a bit of mystery to me. I found a local supplier which was happy to deal in small quantities. As I want this to last, and didn't want rust around my workbench, I cose the more expensive gal. I purchased 1.6m x 3 pieces of onesteel duragal angle and cost me around $60. I was quite happy with the service.