A series resistor (the glow plug ballast resistor) only current limits, the apparent voltage dropped is relative to the current fighting to get through the resistor. Because of this, resistors are terrible regulators for complex systems. You will find that if you add more load after a ballast resistor, that the working voltage for the load will drop. The inverse of going is true as well, and is why if one glow plug fails open, the rest burn out in short order.
Regarding the starter, if you are already current limited, increasing your system voltage will increase your power (power or "watts" is Voltage x Current). In general more voltage translates to higher motor rotation speed, current to torque. And as many a gear reduction starter has done, you can trade speed for torque. Everything else in the chain from battery to flywheel has a part in starting your truck - battery condition and charge, wire condition and size + length, corrosion or wear on contact points. Obviously temperature has a part too, colder batteries can't emit power to a starter as well as warm ones, hot starter motors don't work as well as cool ones, and cold engines are harder to spin than warm ones.
I would recommend keeping both batteries and alternators wired in parallel, but having a dual 12V system - this would double the current available to the starter under 12V keeping your start power the same. If you are hoping to reduce the footprint of your batteries, consider moving to group 31 (roughly half the size of the 6TL battery). This is a modification thread after all...
I have a thread I've been picking at which
goes more into electrical system theory and design that might be a good read.
Personally I'm going the other way - all 24V on the XM1027, and for the same reason listed above. For the same power level, the circuits are smaller. I also like that I can break into the medium-duty and heavy-duty, European, and military parts streams with a 24V electrical system.
