That's too bad about the frame breaking. That was one of my biggest concerns when I modified my bicycle frame. I am not a professional welder, so I am always scanning my frame with a flashlight to see if there are any cracks. The thought of a failed weld joint at the steerer tube at speed sends shivers down my spine.
I'm sure you'll get it all sorted, but I heard when mig or tig welding, the metal just next to the weld material is actually the most prone to breakage, not the weld itself. Also, I used torch welding with brazing the joints, so not the same as what you are doing, but I internally sleeved all the joints. That seemed to help a lot. Your bike frame tubing is looking kind of thin in that picture.
Good luck, can't wait to here about the first test run,
Gilbert