HDU Foam Board?

[Fishbone]

Hello Everybody.
New to your forum and have a few questions to throw out there for anybody that has legit answers.
Thanks

Am currently rebuilding my 1994 Hewes LT20.
Hit a submerged log a few months back and ended up with a couple of leaking cracks in the hull.
It was later discovered that the stringers are in need of replacing, which is where I am at with the rebuild.

My question is?

Is (HDU) High density urethane sign foam the same material composite companies are selling for building boat stringers?
HDU sign foam is closed cell, comes in 4-40# pvc strengths, is mold resistant, water proof, etc., etc.

From what research I have come up with I'm under the impression I should be using a minimum of 26# HDU foam board for my stringers, 1708 Biaxial woven mat and epoxy resin.
Are these the correct materials I shoud be using for the stringer and bulkhead replacements?