The newer v20s had foam under deck between the stringers and gunnels. Problem is, it eventually holds water. You can certainly add it there if you do stringers. If so, you'll want to make the gunnels watertight, put drain hoses on the rodholders to the bilge, etc. Without the foam, its one giant bilge.
Putting foam in the gunnels won't help much since they won't displace any water until you are full of water.
Also, the fuel compartment deck lid needs to be sealed. If its not sealed, your foam will be soaked and your tank corroded. There is a pvc pipe that carries water under the tank & foam from the forward bilge to the stern.
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