ERF:
I don’t wish to get drawn into the endless permutations expressed here, but would like to make the point that ERF in this period would build a vehicle, including an NGC, to whatever specification the customer demanded (within reason) when they ordered it. In cases where this changed and the transmission was swapped at the dealership before delivery, this was done with full factory backing and the removed gearbox was returned to Sandbach and credited.Engines, gearboxes and final drive ratios were carefully matched to suit individual needs. There are many cases where overdrive RTO gearboxes have been used to replace RT’s in service, only to find that the vehicle is then ‘over geared’ for the torque curve of the engine in it’s operating environment, and it’s fuel consumption rises dramatically.
I’d guess that references to over gearing with an RTO box wouldn’t apply in the case of the NGC’s combination of turbo ■■■■■■■ and final drive ratio ?.Bearing in mind that the factory chose it to run happily enough with the the Gardner 240’s meagre torque output.
Which leaves the question of what did the idea of building it first with the wrong spec then changing it after market at the dealership add to the build/purchase cost.As opposed to the customer just asking for it to be built at the factory with what was needed. 
Realistically it was more likely that no one would want to bother with all the aggro or possible costs and would look elsewhere.For something which effectively wasn’t revving its nuts off at 50 mph especially when that happened to be something as thirsty as a turbo ■■■■■■■ running at 2,000 rpm +.