Thank you all very much.
I'm a lot closer to having the confidence to build up the frame without fear of the 'elegant' weld breaking and I'm willing to consider, if necessary, giving up a couple of inches of paint if it comes to that.
I'm also feeling a little more forgiving of the Shimano dropouts. I've always thought that I'd gladly replace them with Campy 1010/a but maybe breakage is more normal than I though, regardless of brand.
Thanks again. Glad I asked and your responses are much appreciated.