I'm starting to think you're crazy, brother. You caught 30 waves on a board that has no major flaws and you want to fill in the concave? I just don't get it. But if you insist...
Typically you use pour foam to fill in foam, which you then shape and glass over. Since the strength is in the glass, and you're not pounding the bottom of the board with your feet, you only need 2 lb. foam. Higher density foam only adds weight you don't need.
Your project is not an easy or quick one. You should be pulling off the bottom glass to do this, then glassing, hotcoating, and sanding. You need some serious skills to do this, and you risk screwing up your board to the point of no return.
Or... you could go old school, and just use Bondo!