I had a spare hour this afternoon, and itchy fingers, so I set about correcting the too-short smokebox. As far as I can gather - and my OS Nock books on GWR 4-4-0s haven't really helped clarify things - it''s the fact that the boiler is superheated which leads to the longer smokebox on the Birds and Bulldogs. From what I could glean from photos, the chimney stays where it is, and all the extra length is ahead of the smokebox saddle. I eyeballed the necessary extension distance, based on photos of the smokebox front in relation to the front frames. I wasn't concerned to be overly precise as the Airfix/Dapol boiler is already a compromise; its more a question of ending up with something that looks right, even if it's not spot-on dimensionally. And that, of course, will be a subjective judgement.
The handrails were removed (leaving the knobs intact, for the most part) and a compass used to mark a series of cutting points offset from the first boiler band. Using a razor saw, I then cut the smokebox in two, cleaned up the sawn-away areas, and then made a lamination of plastic card to go in the gap. I cut two discs to approximate but slightly over-size, test-fitting them to see if they gave the right spacing, glued them together, then fixed them onto the smokebox front. Once that was dry, I did some trimming until they were reasonably flush with the front, and then glued the whole assembly back onto the rear part of the smokebox, ensuring all was level and parallel. I allowed that to dry, then went back carefully over the joined areas with files and sand paper. And that's where we're at now; I'll add a bit of filler to the join tomorrow and then re-sand, and then it'll be ready for repainting and re-instatement of the handrails, which of course have to made anew as the boiler is now slightly longer.
A bit blurred but it hopefully gives an idea of the extended smokebox and how it alters the character of the loco as a whole.