Peter -
for what it's worth, the top's cracking, and then re-cracking after the repair, was not a workmanship thing, but a design weakness, IMHO. Stress is a real, but often slow, killer, of mechanicals. 90 degree corners aren't good, triangulated support is very good.
I'm betting Don can a) fix the welds, and b) suggest a little support, a few gussetts, or whatever, to improve the design, so that it never cracks again.
and if you're busting T-Tops, how are your knees ?
all the best
Bill