Boxhill along with all Martin Evans's designs I can recall does not have backhead cladding as drawn – but this does not mean you can't fit it.
The boiler drawing does not show and thickening pads and I doubt a professional boiler maker would have deviated from the plans without specific instructions and drawings from you.
The only references to the firedoor in the magazine articles I have scanned through this morning is on the cab interior layout. it appears, along with the latch to be fixed by screws. It is not dimensioned.
As the boiler expansion/hold down angles are shown fitted to holes drilled and tapped into the boiler I suspect the firedoor hinge and latch would be similarly attached – but whether you do this is a decision for you to take.
Remember this boiler was designed nearly 60 years ago and as L.P. Hartley said 'The past is a foreign country – they do things differently there' and screwing things directly into a boiler was the normal practice then.