[Basketcase]

Looks like a good time! Your boat is a tad different than mine as I have the older 4 stringer design but you will find a ton of info if you look at past projects on here. I did a bass boat that had stringers like yours and here is how I did it.

I removed the top of the stringer just like you did and removed the foam. I then put the tops back over the empty stringer and glassed them back down. This maintains the shape pretty well. After that I used a hole saw to cut 2" holes every few feet to pour in the foam. When pouring, the foam expands upwards and towards the open part of the stringer (ideally) and the excess goes out the 2" hole. You can then just cut the holes flush and glass them over. Easy as pie. Just be careful about the amount of foam you pour at once. Its powerful stuff.

2 things to note. There is some debate about using 2 or 4 lb foam in there. I suggest to do your own research. Also, now is a good time to think about raising your floor. I did mine by adding height to the stringers. If I were to do it over, I would add the height to the bottom of the floor before placing it in the boat. Seems to me like it would be much easier to do it that way.

Good luck with the project. You are in the right place now. These guys will steer you right and are always willing to help. You can learn a ton just by spending a couple hundred hours searching old projects. Enjoy!