Better to go to 12V.
The 6V system just won't charge an 8V batt.
It will for a while (few days/week) then one or the other (gen-reg) burns up.
The system will eat all the lamps too.
The block heat & dual battery will work okay.
A 12V batt will 'jump' the 6V system via a solenoid and battery in the trunk as a temp fix till you can do the conversion.
Also; 12V positive ground is a little tricky, but not that hard or expensive.
Good luck.
