[Aerkuld]

The older MGB's are, I thought, positive earth, 12v electrics. They are 6v batteries, but they sit in separate compartments on either side of the transmission tunnel, under the back 'seat'. The two 6v batteries are connected in series to give 12v, NOT parallel to give 6v.
A12v battery should be fine to jump it, but you'll want to go to one of the terminals on each of the two batteries. It should be easy to tell which battery terminal connects to ground, which connects to the electrical system on the other battery, and which is the connection between the two batteries.
As others have said, make sure it hasn't been converted to negative earth. It was quite commonly done and, from what I recall, not too difficult.