I tried an 8 volt battery once and I tell you never again will I ever use one. Yes you have to tweak the Reg to put out 9.2 volts this can be tricky. reason being is they the Regulators have to be at an constant ideal temp, plus now your Gen will work harder to produce the voltage necessary to keep up with the battery demand.
There is not a damm thing wrong with the 6 Volt system. i see quite a few using the 12 volt cables, wrong. Use heavy wire Cable I use and make my own when necessary from welding cable Guage 001.
Also I use an Agri Battery the ones for Farm use they fit just nice, and the one i use has 940 Cold cranking amps. Make sure you have good clean ground and charge your battery at least half way through the winter months. Make suer the Starter is in good working order, and all should work just fine.
We use to have winters a lot colder than we have lately and they did the job back then. Worst thing you can do is leave a Battery sitting still, it will destroy itself in a matter of time. The acid in it will actually eat away at the plates rendering it useless.
As for leaving a Battery sitting on a cement floor, as some will tell you will shorten the life of it. That is a lot of B/S, take a look at the bottom its made of rubber. What will happen is that the floor being cold will do the same as if it were sitting in a cold vehicle over the winter months.
I take mine out of the truck and just for the sake of it place it on a piece of wood, but do charge it once during the winter months, and give it a bit of a shake to move the acid around in it.
So for what I have said above is all from experience not here say, also advice from people in the know. One other point of interest if you ever have a Starter rebuilt make sure that the rebuilder installs 6 Volt field coils there is a difference.