Casey,
sorry to hear of your troubles, my experience of "Willi" has been 3 boiler failures, all at the vertical flue joints over the Burner..it seems its adifficult boiler to make, and the water gauge needs careful watching to ensure the joints are always covered by water. Im not suggesting yours has run short of water.. as we had failures in which Im certain the water level had been maintained, just that it seems vunerable in the joint area.
Taperpin.