pedal box help

[robo49]

I have started to make my peadl box but I just cant, get my head round the books dimensions on page 54
the back plate is 268mm wide plus 16 mm for a bend = 284 mm and this plate fits between U6 and U7 then bolts flush to to CP 16 .
The gap between U6 and U7 is 260mm on my chassis
can I just trim the back plate down ? also what is the bend for ?

cheers Rob