So, I plucked up the courage to slice the other side of 125 almost in two, except for a sliver of a cantrail along the top.
I won't pretend it went like clockwork.
As soon as I had removed the section of lower bodyside I decided that the gap didn't look quite big enough and I set about relocating the window pillar on the right, which was not as simple as it sounds because by this point it already had all the beading in place.
That had to be carefully peeled off before chopping out the pillar and moving it a mm or so along the way.
Satisfied, at last, with the positioning I built up a section behind and formed a doorway.
Here's how the finished side looks from the front.
And from the back you can see how far into the carriage body this doorway is set.
I shall turn my attention to the ends next, which are not identical.
For the Caernarfon end I can use a set of standard double door superbarn castings but the other, which is a regular, full-width affair, will be scratch built in styrene.