I have some very good news to report.
David Foulkes of Steam Era Models (ie manufacturer of the J Class kit) has very kindly had the artwork associated with the frame etches modified to broad gauge (18.37mm) and made available to me. Key component that are directly affected by the conversion, including the side frames, frame spacers, pony truck, and head stocks have been modified.
Furthermore, an allowance has been made for the addition of CSB compensation, including holes for pivot points and clearance for the wire in the frame spacers. Finally, the coupling rods have been reworked into three separate sections (as per the prototype), to enable the CBS compensation to function correctly.
I and the owner of the second kit are very thankfully for this generous gesture from David at Steam Era Models. It was as unexpected as it is appreciated.
The artwork has been forward to Tony Crennan of Model Etch for review and an estimate.
While this will slightly delay the construction of the frames, it is a most welcome one! Besides, I have plenty of other aspects (ie everything above the foot plate) to push on with.