After a little head scratching, I decided that there wasn't any way I could accurately reduce the width of the frame spacers fitted to the chassis, it would be a lot easier to make some new ones up from sheet nickel silver.
First job, remove the old ones - I could have attacked them with heat, but wary of disturbing the bearings, I opted to cut them in half with a piercing saw. This gave me much better access to the soldered joints, which were heated and the errant metal removed, followed by a good clean up with a file.
Then I cut some strips of nickel 10.5mm wide using an Olfa cutter and ruler. Even I can get things reasonably accurate with those tools.
Using the Hobby Holidays jig, the sides were lined up and the spacers fitted between them. I know the axles are in the right place relative to each other so the spacers don't need to slot into the chassis sides, I can pretty much fit them where I see fit.
This time the axles stick out both sides of the chassis. It even sits flat on a mirror when fitted with wheels. Success? Well they look like they are a bit proud of the sides. But I can shim this with some washers to reduce the side-play.
Except that the motor won't fit between the frames, and these are deep enough that this is precisely what it has to do. I did contemplate putting cut-outs in the frame sides to accommodate the motor, but there was really only one thing to do...