My 59 3800 with a 235 has a 12 volt stomp starter, so it's not possible to make that distinction. Start out with a good cleaning of all the terminals from the battery to the starter, and back along the ground path. Such things as headlight bulbs will have a voltage designation, or they'll burn very dimly if you try to burn a 12V bulb with 6 volts. It sounds like you've got a 12V conversion already. There should be a 12 volt marking on the generator ID tag if it's been swapped out.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
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