I've used various solutions on all the, er, classics ( bangers ) that I have owned - leak fix by plastic padding is probably one of the better ones...
I've had the same problem with inlets and outlets corroding away and used the above to secure whats left of the rusty parts !. Obviously, the repair lasts longer the less 'longterm' contact with petrol it has - ie if you can keep the fuel level below the repair.....although all my tanks seem to pin hole in the bottom corners...
Ultimately the rust process continues within the repair and eventually the repair will open up again if the rust can swell up enough.
cheers
ash