When I made my outer tubes with the tabs I did this...
first drill the base-mounting hole in all the tabs while they are still straight, do your math to figure out where you want them, you want the hole to be inline with the center of the tube when it is bent and welded
bend all the tabs at the exact same time, using a rectangular metal bar and some clamps, clamp all the metal tabs to the metal table on the band saw, all exactly even in a line, then bend them all up against the clamped metal bar 90 degrees, then re-position them and bend them all 45 degrees, then re-position them and bend them 45 degrees again to make the right shape
then i had a piece of steel rod, made on the lathe, that would slide nicely inside the outer tube, 0.625", then it had a reduce diameter at one end to 0.250", this rod would slide inside the outer tube and the end would slide into the hole in the bent tab, thus aligning it correctly, clamp them together and weld them, use this rod to check all the alignment
if you don't want to use spacers you have to do a perfect job or the tubes will touch, of course using spacers is easy and probably wont change much for performance