Yes, it's possible to rework a V-8 starter for 6 volts. Any good starter/alternator rebuilder worthy of the name should be able to do it. Olds and Cadillac ran V-8's on 6 volts from 1949 to 54, IIRC. No, it's not a good idea. Cranking an engine with the compression ratio of the average small block V8 would put a lot of load on a 6V battery, unless you're willing to run one of the big batteries we used to use in banks of 4 on the big Diesel rigs. It definitely won't fit in the normal location for a battery on a stovebolt. You'll also be stuck with a relatively low-voltage point-type ignition system, because I can't think of anybody who makes a 6 volt HEI system. Bad idea!
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
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