I use a single size 26, 12 volt battery (from K-Mart) in my ’59 MGA in the right-hand side. The other battery holder is used to hold a spare parts bin. MGAs were originally positive ground, if yours still is, then the positive battery terminal is grounded and the negative terminal is connected to the lead that comes back from the starter switch. You had better check how the coil is wired to determine if your car is positive ground. A later coil will have “+” and “-“ terminals. On a positive ground car the “+” terminal will be connected to the distributor.