Just so there's no misunderstanding,
You will not be the first member to paint (cold tar) then foam in a new tank. I stated earlier I hadn't seen the fiberglass method but just to offer another option at this stage of the game, you may want to consider just removing the glass from the tank top, leaving the foam covered. If possible, then paint the top. Cold tar is but one option you could use.
The paint on the bottom should offer some buffer between the tank and rubber (if you subscribe to the carbon theory) and keeping the water out (via the tube) of the coffin between bulkheads should help considerably. Remember that not only the foam but the paint itself offers insulation to the tank and therefore greatly reduces the forming of condensation on its outer surface. My opinion is the bulk of corrosion seen on these (aluminum) tanks comes from water that enters the coffin rather than condensation on the surface but that's another story.
Lastly, I would think you could get a fair amount of time out of this install so the the question will be how long do you plan on keeping the hull?
Give it some thought. Only you know how the builder/installer will react and it's not the end of the world as we know it. However you end up, please use the welded tank tab (designed for same) as the bonding point.
Good luck.
