When I replaced the stringers in my 25 Grady few years back I did a lot of research.. The answer is do it right, buy the two part foam 4 Pound. Yes it is pricey but doing it right will save in the long run..
Google Marine foam, there is a place in Florida that was much less than others, I called them because I questioned it, very nice people explain the entire process to me and why to use the 4 Pound. I bought and was great .
I replaced stringers myself.. big job.. all winter two years ago..