I like the idea of shrinking the flange onto the tube. I can easily
machine the flange I.D. to be a couple of thousands smaller than the
tube O.D. Then I could use a hotplate to heat up the flange then slip
it into place and let cool. I could then put some very small tack
welds in place.

How much undersized would you recommend making the flange I.D. compared
to the tube O.D. for a permanent fit with no chance of slippage?

If I can't true up my existing assembly, I will go this route on the
next one. Thanks.