A small amount of progress today as I dragged myself away from the PS3.
I decided that the grinder option was ridiculous so my old razor saw was offered up as a sacrifice.
Chop, chop, chop in the right places!
This photo shows where material needs to be removed - right at the front of the chassis, and a section by the front driving wheel (which will likely be trickier)
and this photo shows how this relates to the rest of the body.
Once the body fit and mounting is sorted I can sort out the steam pipes, glue it together, and then work on mounting the cylinders and valve gear.