So, I can make some new busbars on the bandsaw (if needed) out of
some suitable brass (?) sheet, once I know a std MCB will fit
reasonably.

You should be able to buy replacement DIN rails, if the amount supplied
isn't enough. If you do have a full length one already, it can be simply cut
in half to insert the RCD. Then you just need to sort out the neutrals. I
don't know about the GE, but many have two neutral blocks with a link
between. If so, you can simply remove the link and attach each side to the
correct side of the RCD with suitable cable (i.e. 10mm). Alternatively,
throw the RCD and use a non-split unit with an RCBO for the sockets.

Christian.