With small children you will really appreciate the high free board. She's coming along nicely. You got a splash date in mind?
Can you post up a picture? Assuming your tank has no mounting tabs:I can imagine two ways of mounting your tank. One would be bracing it with the starboard as I mentioned, then strapping it down with metal straps insulated with neoprene strips. I might glass some sacrificial wood mounting pads to your stringers, the screw the straps into those wood pads.Otherwise, you might wish to foam the tank in... it'll last a long time if done properly.
The foam you are talking about is called "Great Stuff" and is sold in several varieties, including high expansion, low expansion, and normal. I would go with the original type mounting, with the straps over the tank, and a treated 2x4 anchoring the end. The "Great Stuff" foam may not be gas fume compatible, and also may retain water or stimulate fungus growth. The original straps and tank mount setup lasted 25+ years, so it seems to work. If it ain't broke, why try to fix it? You are right about the tub, when it's sealed up, the only moisture should be a very small amount from condensation. Drill a small hole in the rear lowest part of the tub and you should be good to go. Sounds like you will be on the water in time for the April/May striper run - Good luck!
You can cut and/or bend the original straps to fit your tank - assuming is is smaller than the original. Keep the original rubber straps between the tank and the hold-down straps if you can. Also, regarding wood in the tub, what was the condition of the 2x4 you took out? I would bet it was OK. A similar piece of treated 2x4 will last almost forever in the space that you are putting it in. Cut one to fit, put a coat of paint on it (consider using some bottom paint if you have any around), and don't worry for 30 years.