One thing to consider, at least on shorter length tubes is that as you
weld, the air inside the tube will be expanding with the increasing
temperature, if the tube cannot "breath" a bit it will cause a blow
hole in your bead just as you finish it. If I were doing my project
again I would weld it tight, pressurize it, and make a habit of
checking the gauge for pressure loss on preflight like the aerobatic
guys do. More work but negligible extra weight and great indicator of
cracking.