I know this is an old thread- but did you manage to sort the issue Brian?
The reason why I resurrect this thread (after extensive searching) is .....I have a similar issue!
I have resolved to fabricate new lugs, while strengthening the frame in general.
I have now seen 2 frames with stress fractures in the same place. (see other threads kindly started by Brian)
I think that if anything is to be carried on the rear of the bike regularly- frame strengthening is warrantied. its not a question of "if", but "when" & when is dictated by 2 factors
1.Weight ( the greater the weight; the greater the chance)
2.Time ( the longer one travels with weight; the greater the chance of failure)
...it's a preventative measure.
I recollect that I scrapped another frame and chopped the lugs off of that one.
I hope your fabrication and frame repairs go OK.
Stress fractures are repairable. If the top bars on the rear subframe are rusted it becomes more difficult. This doesn't always become evident until the frame gets shot blasted and you can see the pinholes. The lesson I learnt was to check the shot blasted frame and effect any repairs before it was powder coated.