12 volt Battery choice
When choosing a suitable battery there are two factors to consider. The battery capacity (AH) and the CCA, the cold cranking current.
An electrical system of a standard MGB is rudimentary, it is not packed with modern car electronics or current consuming items like electric windows, seats or air con. With most of us only using our car during fine weather the need for a large capacity battery to support the rigors of winter driving (lights, wipers and in the case of the GT the HRW) is not necessary. What is important is the Cold cranking current (CCA). This is the amount of current a battery can supply when turning the engine over on the starter and is important as the B can be hard to crank and start.
So to choice. Firstly the battery fitted to the later rubber bumper cars (Type 079) will not fit into a chrome bumper battery compartment. There are 3 commonly available batteries types (063, B036 and 202) that will fit as direct replacements and offer significantly improved CCA over the 6V batteries.
The type 063 will fit into the right hand battery compartment. It is a bit of a squeeze but will fit and can be secured using large cable ties.