I'm trying to upgrade the brakes on my partner's 80s Raleigh mixte frame from the old Weinmann side-pulls to a modern set of dual callipers.

As you can see from the photos, the current rear brake is an "upside down" version of the classic side pull, to allow the cable to be run from the long middle tubes below and up to the calliper. My new brake is of course just a standard one where the cable runs out of the top of the calliper. So I'm guessing I should just mount the new brake on the flat metal bridge between the long middle tubes? That would provide the straightest line for the cable...

Which brings me to my problem. The bolt supplied with the new rear brake looks far too long to be able to mount to the thin flat plate. The nut would need to screw on further than the thread of the bolt will allow (see the threadless area on the calliper bolt).



How can I get around this problem?

Should I remove the curved washers (seeing as they won't help in this case) and find some thick washers to take up the surplus space on the bolt? Would this be secure/stiff enough when braking?

Or should I try finding a shorter bolt (I'm not too keen on this as it will involve disassembling a brand new part).

Or should I send this one back and swap for a calliper with a different fixing? If so, what type of dual pivot calliper would fit best?