[sick472]

I can't help you with the measurement, but my approach would be to mock up the assembly of the tube and motor. Then fab up the tab and bolt it to the proper location...then tack it to the broken tang on the tube. Disassemble and finish the weld.

No measurement mistakes and it all lands where it needs to. EDIT: OR cut the existing tang off the tube and make it all new and tack it to the tube with the above approach. That might turn out better so long as you don't burn through the tube.