To move forward, you must sometimes move backward, Bulleid firebox and ash pan.
The kit ash pan flange plate only covers the front 2/3rds, that under the cab is none existent, not a problem until you deviate off the yellow brick road, in addition the side sheets have an odd kink in the upper edge, it's there for a reason, I just haven't found out what that is yet.
Nevertheless the top section needs removing and rebuilding, the flange plate is extended to the rear and new side sheets fitted along with a new floor over the delta track area. That allowed the pipe fixing brackets to be added; they'll be cut back once the pipework and securing clip is fitted.
Then it was time to 3D the new firebox base. In an ideal world it'd be one module, but the cab front/casing rear bulkheads impose and it's a lot of work to hack those back to give clearance this late in the build.
Currently the modules are just held in with double sided tape for visual checks; I can already see that the front section perhaps needs to move forward about 1.0 mm. Plus, the gap twixt firebox and casing is uncomfortably tight to get the pipework up there, so both sections will need their widths reduced by 0.4 mm each side. I've also spotted that the stay bolt heads are not the same height on each module (since corrected and in the printer along with the rest of the tweaks).
That will of course expose more of the flange plate which still has the tab holes for the original metal firebox base, which incidentally is a flat base and the real thing is rounded quite significantly.
The up shot is that for future builds I'll leave off the kit firebox base and fit the 3D version with the rounded base, even without open panels the base at the front is still visible below the casing so the effort will be worth it. I may also get new side sheets flange plates and floor sections etched up as that'll save several hours modifying and scratch building the replacement parts.
Once I'm happy with the placement and fit I'll solder small hoops of wire in the oval holes and then fill with adhesive, that'll bond to the hoops and resin module for a nice strong fix.
The final part will be to add some lagging higher up the side of the firebox and under the base of the boiler, I have a few ideas for that so we;ll see if they work or not. If it does then I'll add a brass belly for the boiler and lag that to cover the gap from front to rear.