Properly wired, the 12 volt system will provide the same voltage to the plugs as a 6 volt system. You just need to get the correct coil and resistor. As far as destroying the engine with hotter spark and the 216 not able to handle it, he's blowing smoke. The 235 block and head were cast from the same metal as the 216 and have been running on 12 volts systems for a long time.
I have no problems with a 6 volt system until it gets cold out. And seeing you're from western Nebraska, I know it can get cold out there. If you run a 12 volt system with a 6 volt starter, your engine will turn over a whole lot faster, needed when it's cold. I've spent some time in the sandhills and have witnessed the effects of -25F temps with strong winds. Bad enough that we would not only plug in a block heater, but wrap a tarp around the tractor engine to keep the wind from sucking out all of the heat. Even with that many times we only had one tractor that would start reliably, a F-20 with a hand crank.
I put a 12 volt 10SI alternator on my 52 four years ale, and have never looked back.
