I read in one of the metal working books that some people braze a big fillet on the outside where the tube goes into the flange. The brazing rod is a little more flexible than the welding material, and has a sort of natural wetting action that makes a good seal. The down side is that once you braze something it can't easily be welded again. The upside is if you do it right it would look really nice (I used to build bikes this way back before everything became carbon fiber and titanium)
Leif
