Battery question

[RSpartano]

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

[KimMG]

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.