Hi Whale,
I spent alot of time looking for a ready made plastic belly tank and didn't find any of them to be satisfactory. What I found was that the angle of the V20 deep V hull was about 22 degrees and all of the available ready made plastic belly tanks had a bottom angle of about 13 - 15 degrees. The ready made plastic belly tanks were Todd or Moeller. The angle of the tanks belly made it that the largest ready made tank that would fit below the floor was 29 gallons. All of the larger belly tanks would have the top of the tank above the floor. How I checked that was by getting the exact dimensions and angles of available replacement tanks and the exact dimensions and angles of the tank well in my V and did an installation simulation on my computer.
Schools suggestion about tank sealer is I think a good one. I called Por 15 about their sealer and found out that their sealer was ethanol proof. They guaranteed that if the tank was prepared properly internally (cleaning & etching) the sealer would permanently adhere to the inside of the tank and the tank would not be affected by ethanol. Por 15's website has full instructions as to how you clean, etch, and seal fuel tanks. The sealer is a tried and true process and has been used in antique car restoration for years. Where do you buy a new exact replacement gas tank for a 1910 Franklin?? Another company that puts out a similar product is KBS Coatings. The cost to fix holes in the aluminum tank and to clean, etch and seal it with sealer would be about $150.
Just my $.02
Geek
http://www.kbs-coatings.com/index.aspx?AFID=7
http://www.stoprust.net/Fuel-System-p-1-c-8.html