Ian,
I've just had a quick look through the info I have to hand. To sum up:
DS3152 was built as 272 in 1889, to works order X7. It was built by Drummond to the original Adams specifications so there are no significant modifications required to the kit; it has lever reverse and the longer boiler. Bizarrely it also appears to not have any wheel balance weights in any of the photos I have of it. That makes the main error in the kit the long O2-style spring hangers, you can replace them with 3-axles worth of Laurie's part 27-42. The photo I linked to in post #48 confirms this quite nicely as the spring hangers are entirely absent.
In terms of cab details, and with no G6 still in existence, it's worth looking at the O2 'Calbourne' as preserved on the Isle of Wight. Traps to avoid are that the 02 is air brake, so there's no vacuum ejector. The air brake handle seems to be mounted on the right hand cab wall and there's an extra handwheel - the governor/steam valve for the Westinghouse pump.
In terms of the brake valve vs. combined ejector, Laurie's notes in with the cab backhead point you in the right direction: When vacuum (rather than steam) brake was adopted for Adams locos the combined ejector was fitted and the lever-style brake valve removed. The additional valve over the firebox on the driver's side is the steam valve to supply the combination ejector; you'll need to find or make something suitable.
I've had a look at the position of the firedoor handles on the GA, photos of the O2 and the LG parts. The latter are correct, but it appears that the lower fulcrum should be prototypically mounted below floor height. You'll then find it'll all work quite nicely. The pics (below) should help with your finishing of this part. The major omission from the LG backhead set is the plethora of handwheels required for the blower, two injector steam valves and ejector steam valve.
Other details to note, and an error on my part for which I apologise: It appears that DS3152 did indeed maintain a number of Adams steam fittings, the whistle manifold is therefore a combined casting with the injector steam valves; assuming you've already got an Adams whistle you should be able to use it as supplied. In the photo links (below) be aware that the O2 has an organ-pipe whistle rather than the style you need, but the casting and plumbing it sits on does apply to your model.
I haven't got a clear view of DS3152's injectors, which are tucked up under the tank between the middle and rear drivers, so can't confirm whether they're Adams or Drummond type; you would get away with either, or just a carefully-shaped length of wire!
Piccies, of 'Calbourne':
http://www.flickr.com/photos/65614637@N00/5221840406
http://locoyard.files.wordpress.com...t-steam-railway-havenstreet-w24-calbourne.jpg
http://locoyard.files.wordpress.com...t-steam-railway-havenstreet-w24-calbourne.jpg
Steph