Hi Tosh,
If all of the measurements are as per the drawings as you say, then the only thing I can suggest is a couple of 1/2" spacers (one either side) to keep the brake bar central.
The tub must be right as it was bought in from Richard, I've just checked the manual, and as you say, the metalwork is 16" and 7/8" (430 mm) wide, so that's OK.
If you look closely at the diagram on page 41 of the manual, in the bottom left corner of the page it shows a M10 bolt passing through the batten and the Rear Inner Wing panel, secured with a nut. Then the brake bar goes on, and is held in place with a 'Nyloc' nut, to allow the brake bar to swing freely.
Simple thing to miss on such comprehensive diagrams, I only noticed it on the second look.
Hope this has helped,
Keep Busy,
Regards,
Mark.
P.S. is it ready yet dad??