The following applies to the 3N71B transmision & was gleaned from my B210 factory manual. If the 1200 trans is the same model [& i suspect that it is] then it is relevant

Manualising the transmision involves making some adjustments to the govornor, modulator & gear selection control circuits on the trans. ALL of these parts seem to be accessable with the trans still in the car, even if it means pulling the extention housing.
The valve body assembly is accessable by pulling the pan & it can then be removed entirely from underneath if necessary.

A clever auto trasns bloke will be able to give you some sort of answer, & maybee even an estimate if he can get a look at your workshop manual first as the factory book is extremely comprehensive.
Removal of the trans should only be necessary if you need to overhaul the mechanical parts of the driveline within the transmision case itself.

Another alternative is to change manifolds & run one DelOrto instead. When correctly tuned, it will likely run better vacuum figures & the trans will work better, but i suspect that one carb doesn't provide the right image in the engine bay for most people.