I've expressed my opinion on the subject several times. While the starter is capable of handling the overload of an 8V battery, if you do everything necessary to keep the battery charged properly, the rest of the system will be running on the ragged edge of over-voltage. The headlights in particular will be on the verge of failure all the time, and the generator and regulator will be running at the extreme upper limits of their operating range.

A much more effective approach would be to use two 6V batteries in a series/parallel system where the charging system and other circuits operate on 6 volts during normal running, and the batteries get connected in series during cranking to put 12V on the starter. Really, it's much less expensive to just fix the 6V system properly.
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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