Several GM cars had the same problem in th 60's including one of mine. But old volkswagon people know of a starter booster kit for the 6 volt models. The kit consists of a continueous duty solinoid (A Ford starter solinoid) and it's wired up like this:

10 gauge wire from starter battery to left side of solinoid.
10 gauge wire from the right solinoid terminal to the starter's Start terminal.
Start wire from ignition switch goes to S terminal of Ford solinoid.

I did this to a buddies International Scout that had the same problem. It worked fine and he said he didn't have to turn the key as hard to start it.

If you think of the princible, your starter solinoid is an electro-magnet. If you don't have the amperage to energize it, it won't work. The actual power loss may be in your wiring harness. The starter booster gives you battery amperage via the 10 gauge wire direct via the Ford solinoid. I think GM's problem in the 60's was a 16 gauge S wire in their wiring harness.

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1984 280SEL, 62,000 miles
Euro model in the USA
1983 300D (Totaled out 10/2004)
New Factory Mint Green paint
Palomino int
1982 240D (Steak dinner for two)