Battery question

I have a 1974 mgb with 2, 6 volt batteries. The battery behind the driver seems to build up and foul on my ground terminal (Neg ground). When I go to start the car and it won't, I realize the problem. I need to diconnect the cable from the battery and clean both the cable and the battery terminal. This happens maybe 4 times a year at the worst time. With time I end up with a cable connection to large and a battery terminal to small, having to replace them both. Any ideas on how to solve this problem? Going to the conversion 12 volt battery is an option but I would like to keep the car as original as possible. Thanks up front for the ideas.
Rocco P. Spartano

The ground cable may be corroded under the insulation.
Check to make sure the contacts on the body are clean.
Use dialectric grease on all the connections.