Iain,
I have measured some parts and this is what you need to make.
4 off spacers from 1.5″ hex bar. The main hex is 3.0725″ long with a 3/4″ BSF thread on one end an additional 1.25″ long and the other end tapped 3/4″ BSF 1.75″ deep.
The side frame is removed and the wheels reversed to the wide position.
The spacers fit on the front and back axles so the side frame can be slipped back on and retained by the original axle nuts.
The original design then had some spacer blocks that sandwich between the side frame and main chassis where the 2 long bolts are. This can be a fabrication of a heavy channel so that 4 short 1/2″ bolts can bolt it to the chassis then the long bolts can be refitted to hold the side frame to the outer part of the channel.
Trust this is clear and helps.
Stay Safe.
Neil
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