[Growley bear]

The 6v starter is two series parallel circuits. Two separate circuits that run in parallel with each other and in series with the battery. When converting to 12v you end up with one series circuit which increases the circuit resistance there by eliminating some of the initial shock. More voltage/less amperage. 12 on 6 causes a slam when engaging the starter; the Bendix is not designed for this sort of abuse. The initial torque/amperage draw is tremendous.
I have a starter armature somewhere that was run on 12v for many years; the spring stretched and wore a long helical groove in the shaft.
I talked a friend who was running 12v with 6v starter; I built a 12v starter for him and he is now a firm believer in 12 for 12.
To answer your question; if your starter and electrical system are in good shape and you install a 12v battery your engine should turn over as if the spark plugs have been removed. And 6v on a 12v starter would act like a dead or very low battery.