If you know that 6 volt systems are really bad, what is your question? I believe you have answered it yourself. I wouldn't change it unless adding air conditioning or modern sound system.

My experience (having driven my 6 volt Essex since 1965) is that 6 volts work well as long as the connections are good ( no corrosion) & the wires are of an appropriate thickness. I recommend a "battery tender" be connected when the car is not going to be used within a week or so, this will prolong battery life significantly..

Are you having problems with your car? Problems that a 12-volt changeover would solve?
Really, speaking for myself, I have been driving my '37 for 48 years with a six volt system, with no problems. I DID have a problem when I first bought it. I salvaged a battery cable from a derelict '57 Plymouth, reasoning that since it was a 12-volt cable it should work twice as well as a six volt one. Big mistake.

I suspect that the reason a lot of people change to a 12-volt system is because there is some problem in the electrical system that's putting a strain on it. Changing to 12 volts may seem to remedy the situation by making the starter peppy-er but you haven't really remedied the problem. Fix the problem correctly, and the old 6-volt system should work fine.

Follow Davids suggestions and be sure you get a good quality battery with plenty of Cold Cranking Amps.

you can use a 8volt battery

If you have a good 6 voltbattery, The correct size 6 volt cables, and your starter and generator are all working properly there is NO
Reason to switch to 12 volts. Unless you are adding air conditioning. I have cars still running 6 volts that I’ve had for over 30 years.

Hi Bernie, I replaced all the old 6 volt battery cables with 1 O copper cables and new cable ends. Cleaned all mounting surfaces and bolts of oil, paint and rust. I did a little over kill by adding an extra 1 0 ground cable from grounding bolt direct to starter. Not really necessary but didn't hurt. Installed highest cold cranking amp red Optima 6 volt batter. Also, replaced generator/voltage regulator problems with 6 volt positive ground alternator from CHEAP WILLIIE'S 620-228-1477 ask for Bill if interested. Car now stars instant I turned starter key. Lee

Clearly up to you what you want to do. My 39 still has its 6 volt system, generator and ignition points and runs just fine. As others have stated if your gonna add air then yes 12 volt is the way to go. But otherwise I personally would leave it as is.

I agree that 6V cars run fine on 6V if the electrical connections are proper. If you stay with 6V, I recommend a 6V Optima battery, which I installed in my 54 Hornet. The Optima made my 54 Hornet turn over almost like a 12 V battery. I bought the Optima battery about 10 years ago and it is still working well, but now installed in my 51 Pontiac. The reason I converted my 54 Hornet to 12 V is that in 2013 my son and I drove my 6V 51 Pontiac from Seattle to Dallas (details are at https://fugutabetai.com/?postid=520). First the "new" fuel pump gave out in the mountains in Montana, probably due to the ethanol content of the fuel. We found a 6V electric pump with lots of trouble and delay. Then in Albuquerque, the 6V generator gave out. So in preparation to driving my Hornet from Washington State to Dallas, I converted the Hudson to 12V so it would be easier to get parts in case of a breakdown. The Hudson made the trip to Dallas without any breakdowns in 2016. I likely will convert the 51 Pontiac to 12V, since I like to take long trips...

12v conversion is very involved, so yes, make sure you have good reason
My reason was for A/C and ease of getting a jump in case of breakdown
I have additional benefits of modern fuse block with blade fuses, modern charging jacks for iphones, bright lights, and instant starts
Need 12v conversion on starter and radio, and several resistors or voltage regulators to run accessories.
I'm still working on getting gauges to read correctly with 12v->6v reduction

Before you convert to 12 volts make sure your battery is good, all cable connections from the battery are tight and corrosion free. The main issue with 6 volts is that when a starter ages it may draw so much current that the available voltage to the coil is greatly reduced. In my '54 Hornet I have installed a Sure-Start box which provides 12 volts to the coil when engaging the starter, and reverts to 6 volts once the engine is running.