If you are just adding a bit to the bow, it won't matter which foam you use since the glass and epoxy will be taking the loads. I built a pair of amas "surfboard style" with a stringer between stacked and glued pink insulation. Crappy foam? Yes. As long as the bond doesn't give way, it works great though. Think of all the surfboards that were made with less than stellar foams. Some current epoxy boards are made with what amounts to styrafoam. Your skin will be taking the loads if you don't modify the underlying hull other than roughing it up for bonding the foam and glass to it. That said, you don't need much and might be able to get some klegecell or Airex cheap.

Dan