I've drilled it out to fit the gearbox, as I only have access to a mill without a readout the easiest way was to reference everything from a corner, this drawing shows the hole positions.
So I now have the mount plate made, and all the holes seem to line up quite well. Turned a couple of locators to help with final alignment onto the gearbox
Also turned the shaft to spline adapter, although I've simplified it a bit when I turned it as I wasn't able to do the rotated arcs on the manual mill, but the parts seem to fit together well.
So I've offered up the gearbox onto the motor with the parts and it looks ok, the thinner than originally planned disk based adapter does mean the end of the spline hits on the end of the motor shaft by 2mm but I'll cut off the end of the spline. Having the motor closer to the gearbox will be a big benefit to fitting the whole lot into the frame as there is very little space between the motor and frame.