I'm with the otheres on this, a cut off switch inside the car. Preferably somewhere and of a type that can't be accidentally hit, someone I used to know had a push button type cutoff in corner in front of the parcel shelf (that the batteries are under) occasionally the car cutout after heavy braking when something on the shelf slid of onto the switch (not a problem with most modern switches).
Also if you want to thief proof the batteries I've seen a mod on a couple of cars in the UK where they have boxed in the battery frames with welded sheet steel panels and louvered on the sides to vent the hydrogen that the batteries give off.
I have also seen at least one car where they have moved the battery into the engine bay in front of the heater and one one (pre 1974 car) where it was put in a frame in front of the rad.
If leaving them where they are then use maintenece free batteries (which is of course a given).
For the record in my 68 MGC I run two 12 volt batteries in paralell to give the starter a bit more amperage, cheaper than two 6 volts and takes up the same amount of space. not a problem in a B
but the 6 need a bit more juice to kick it over, particularly when it has just had an engine rebuild.
regards
Tom