Hi Kev,
I've only just found your thread and it's interesting to see that you're encountering similar problems with the build, after getting 3/4 of a Mitchell 517 build completed this is an eye opener to me, it is also the first build in decades that I have felt like throwing at the wall !
You will have to plan the spacers pretty soon as you may encounter problems with the space between the outer frames and wheels ( assuming you are using Slaters like me ? ) which would be to S7 standards if you opt for the 28mm ones.
I'm a few days into the build and also noted that the boiler wrapper is approx. 1mm too long so had to alter it, I assumed, wrongly, that because all the other etches fitted pretty well then this would be fine too.
If you haven't done so already then place the splashers on the front and scribe a line onto the boiler. I made a mistake here which meant that the front driving wheels wouldn't fit into the splashers. The etch was removed, flattened and altered then re-rolled.
I would recommend lots of dry runs before committing and totally agree that the instructions are sparse to say the least with several of the numbered etches not referred to in the written instruction and the cage to the firebox is incorrectly show in the photographs. I now understand why the late Raymond Walley remade his.
Here is where I have got to so far and also showing the alterations to the boiler in order to accommodate the wheels which will be fitted using Slaters hornblocks.
Not the best of photographs but I think you will understand what it refers to.
G
p.s. note the two alternative footplate etches are different widths which will affect the splashers in the cab.