Just to add a little to the posts above.
The East Anglian did have stock constructed specifically for the service on the GEML but this was six coaches of conventional construction and not streamlined or articulated so it bore little resemblance to the ECML streamlined trains. The two B17s were painted green pre war, like some of the A4s. As well as being shorter, the steam pipes were visible as bulges above the cylinders so that and the problems mentioned by 34theletter... suggests the Golden Arrow resin bodies might be an easier route than hacking at an A4. The streamlined casing added 3.5 tons to the weight of the locomotives but since the time allowed between London and Norwich was generous, this hardly mattered. I think most people accept this was an LNER window dressing exercise but there is little doubt the LNER publicity dept. knew what it was about and they managed to charge a supplement for this service (yeah, I know, some people paid extra to go to Norwich! ).