That solder is going to be really ugly, have you considered phosphorous
bronze? Good color match, much stronger and tougher joint too.

Try a plumbing wholesaler.. 1-1/2" Cu Male adapter and an inch and a
half Brass Floor Flange.
Flanges aren't all that square, but chuck 'em up in the lathe with the
male adaper soldered on and you can come very close....

Bill in Phx