Just a suggestion....I always fill my tank till it overflows. I would suggest using parbond to seal the backside around the fill neck, the vent neck and the 2 rivets that keep the fill neck strap in place. This will prevent any water from back-flowing between the walls. Not sealing won't allow a lot of water in there, but I don't even like a few drops getting to the floor and c-channel.
"If the women don't find you handsome, they should at least find you handy." - Red Green