Tom, you ask quite a series of questions, so here goes:
"Can a shop modify a generator to charge an 8 volt battery? I’m assuming that “working harder” means that it:
"1. May not fully charge an 8V? Or can fully charge the battery but it will take longer?"
The generator does not require modification, but the voltage regulator needs to be reset in order to charge an 8V battery. It will stress the generator a bit more.
A generator and its companion voltage regulator set for a 6V system will not charge an 8V battery, since the nominal setting of the regulator is below battery voltage. The 6V system voltage regulator is typically set to 7.5V, plus or minus just a few tenths (minimum 6.9V per old Delco info - see first image, p.6 of Delco 1R-116A, dated 8-1-60). An "8V" battery is actually at 8.4V when fully charged, similar to a "6V battery" being at 6.3V when fully charged. The voltage regulator would need to be adjusted to greater than about 9V, in order to fully charge the 8V battery.
"2. Can charge an 8V but it strains the windings or creates additional heat?"
Yes, the generator can support charging an 8V battery once the voltage regulator is adjusted to set the system voltage higher than the battery voltage. That may be a bit more stress on the generator, and probably produces more heat in its windings.
"How is the Regulator adjusted to 9.5 volts? Why 9.5V?"
The relays in the regulator can be adjusted to set the voltage higher, and 9.5-9.6 is sufficient to charge an 8V battery. It's a bit fiddly, and takes some test equipment to do the adjustment correctly. The procedures are in the old Delco literature on how to properly set a V/R (Second attachment, full copy of Delco 1R-116A in .pdf format). The charging voltage, about 9.5V, is chosen so that the system voltage is greater than the battery voltage (at full charge of 8.4V), and high enough so that the charging rate is reasonable. A battery only charges if the voltage applied to it is higher than what it produces.
"8V on 6 volt bulbs: I understand they (6V) will burn brighter on 8V but, what is the downside? Do they burn hotter and shorten the lifespan? Is there such a thing as 8V bulbs?"
The downside of running a "6V" bulb is shorter bulb life. The incandescent bulbs spec'ed for the 6V systems are actually rated for 7.5V or thereabout, since they need to tolerate the higher system voltage set by the V/R. They are fine at 8V. But at the higher system voltage (about 9.5V), if set to properly charge an 8V battery, they will be brighter, but they will have shorter life (considerably hotter filament). An "8V" bulb would need to be rated 9.5-10V for reasonable life in the system. I'm sure there are such things, but I've never researched for socket-compatible bulbs.
"Will a 12V bulb burn dimmer on 8V? Or not work at all?"
Nominal 12V incandescent bulbs could be used, but they will be much dimmer than if properly powered. It's possible that some 12V LED bulbs would work, since they typically have a wider voltage range tolerance. That's something else that would need to be researched, and is probably specific to individual LED bulb brands.
"Will an 8V battery risk damaging my NOS 1946 GMC 6V gauges? They took some effort to acquire. Same goes for 6V heater motor. "What is required to run an 8V battery to avoid the issues of over voltage on 6V gauges and accessories?"
There is some risk, mostly to the fuel gauge, if the gauges are the same as the Chevy gauges (temp and oil pressure mechanical, so system voltage doesn't matter, and an ammeter measures current, and doesn't "care" what the voltage is set to). To be safe, it seems a true Runtz resistor setup could be used with the fuel gauge, similar to what is done for 12V conversion. Having said that, I know quite a few who have run 8V batteries (usually not set to properly charge), without damage to the gauges. Not sure with the heater motor, it may be OK, but will run faster, and the heater speed switch, if so equipped, could overheat. Probably safer to put in an appropriate voltage reducer. YMMV.
"I see online that there are 8V battery chargers and trickle chargers."
Sure, 8V batteries were commonly used in tractors, and some golf carts. When I was a teenager, my friends and I would "borrow" tractor batteries from one of our Dads or Uncles, to get our cars to start in the winter (upstate NY). We never bothered to worry about charging - we'd just swap for the next one when the cranking got slow. Risk to the system? Never gave it a thought. Dumb kids.
As a post-script of sorts, IMO, the 8V conversion is more problem than it solves. I'm running a 6V system on my '53, and with appropriate cables, grounding, and some maintenance, all is fine. If I were to convert, I wouldn't bother with 8V, but would make the jump to 12V. Conversion complexity is about the same, and 12V components are readily available. I just haven't seen the need to move from 6V for my purposes.
