: I'm about to start stitching together the hull for my CLC Shearwater 16
: Hybrid. I'm looking ahead to foot bracing, and considering using custom
: foam blocks against the front bulkhead. Is that 4mm glassed-in bulkhead
: strong enough to take the pressure? If not, how about with a stiffener
: and/or extra perimeter glass on the back side? Thanks much.

Depends a bit on use. Will you be standing it on its nose in surf? Then you will need to fibreglass it back and front, fillet it around the perimeter (in forward compartment) and cover that with several layers of fibreglass bandage. You generate quite a bit of pressure standing on it. If you don't anticipate doing that or something like that you can go with something lighter.
JohnK