Hi Ed.
There is a guy on ebay who is parting out a 337D. I think the tanks are the same. Here is a link to one of his auctions for a different part, but you can find his contact information there.
http://www.ebay.com/itm/Cessna-337D-...17f77f&vxp=mtr
I bought an inboard flap last year off of another seller on ebay who was also parting out a plane. I think the sellers ebay name was baspartsales. You might try to track that down as well.
I am concerned that your shop "thinks" they have cracked a fitting. If it is the inboard tank, between the boom and the fuselage, there are only four 4 tank penetrations, plus the hole for the fuel level sender. Three of those penetrations are only tubes for a hose and clamp, and the fourth is a threaded boss for a compression fitting. If they have the tank out, it should be a simple matter to fill it with water and see if it is leaking or not.
I think I know the shop you are using, and they are good. But if the tank is in fact damaged, I think I would investigate sending it out to a shop that specialized in repairing aluminum fuel tanks. I find it hard to believe that your tank would not be repairable.
I feel your pain. Last year, I had a stain at the wing root. Fuel was leaking around the 40 year old cork gasket at the fuel sender on the inboard tank. I replaced the gasket, and the leak stopped for almost a year. When it started again, I tracked it down to the clamp around the 1 1/2" hose leading from the middle tank to the inboard tank. The fuel hose had shrunk from age. You can't get to the clamp without opening up the whole top of the wing. When I did, I found that most of the cork anti-chafe strips had migrated out from underneath the tanks. So I elected to take out the tanks, replace the cork strips, and replace all of the 40 year old hoses. I'm still working on it. It is the hardest thing I've ever done, but it will be worth it when I'm done. I hope.