There are several things to consider wen building the frame,
Cut each tube a little longer than the exact length, then round the edge with a round file. That way the tube meeting the other tube cups it. It's stronger and easier on the welder.
Keep an eye on the front seat tube, when some of my tandems were welded, there was filler in the seat tube, that limited the amount of length the seat post could go down. Talk to your welder about this.
Use beefy tubing to connect bb-bb. I used a cut section of downtube from another bike.
Have fun. I've done it several times and I have no regrets. They're plenty strong and ride fine.