70 coronet 8.75 in 70 dart

You are in the right direction...

If you want to keep your SBP you would need to get some A body axles or have custom made axles. IIRC only A bodies used the SBP everything else used the BBP.

Thr perches would need to be moved inboard 1/2 inch each side.

The backing plates need to match the axles, A body has a different back space than the rest. So if you go cut down axles you need the backing plates that came with it,

Unless you are going to do the cutting and welding, narrowing and resplining yourself, I think even with shipping to the UK buying the parts used or from DR. Diff to get matched SBP A Body housing, brakes, and axles would end up being cheaper