Have you done any research? This has been done before and the hot rod guys know more about it than the people on this forum. I assume when this is "done right" everything replaced and upgraded to 12V. The gauges are probably the trickiest part (if you want to retain the original appearance).
This is "not easy" for things that operate at more than about 1 Amp. You can look for "switching" (AKA switchmode) voltage regulators. Switching regulators can handle more current than linear regulators and they are nearly 100% efficient (so they don't get as hot and they can generally handle more power).
You can't use a regulator with analog electrical gauges because of course the voltage is not constant. A series resistor might work.
With such items as the heater and the roof fan,
I assume that's not an electric heater... You just need to power the blower, right? Do you know the current (Amps) rating on the motors? Do you have a multimeter to measure the current? (Be careful because you might blow the fuse in your meter.)
Chassis mounting resistor 850F10RE
What's that for? If you're trying to drop the voltage you can make a voltage divider where the "bottom" resistor is replaced by the load (such as the fan). But, you'll have to know the effective resistance of the load and it has to be constant. That means two loads can't share a resistor because if you switch-off one motor the voltage to the other will go-up.
It's also inefficient because with a 50/50 voltage divider the resistor has to dissipate the same power as the load. And for a 50/50 voltage reduction the series resistance has to match the load resistance.
Note that you can't measure the resistance of a motor (or light bulb) directly with a multimeter because it will change when the motor is running. You have to measure the current and calculate the resistance (Ohm's Law).
how would I go about reducing current to 6v so they can operate?
Current is the flow of electricity (Amps or milliamps).
Voltage is "electrical pressure" (often called "potential").
Resistance is the resistance to current flow. More resistance = less current.
The water-flow analogy works reasonably well - The water pressure is (approximately) constant. A skinny pipe has more resistance than a fat pipe so more water flows through a fat pipe. The difference is - If you cut a pipe you get zero resistance and water flows-out everywhere. If you cut a wire you get infinite resistance and zero current flows. Also, nothing bad happens with zero water resistance (except maybe a flood). With zero electrical resistance you have a "short circuit", you get excess current and things burn-up (or you blow a fuse or something).
Ohm's Law defines the relationship between voltage, resistance, and current (Current - Voltage/Resistance) so if you know two you can calculate the 3rd unknown value.
Power is Voltage x Current. With a little algebra you can calculate the current and/or resistance for a light bulb if you know the wattage.
Can I use the 6v distribution box that's on the bulkhead of the dodge? Or would you bypass it?
Connections & fuses "don't care" about the voltage, but what's the condition?
I'm aware the starter will work happily on 12v at short bursts.
OK, but... When you double the voltage you also double the current. That's 4 times the power (Wattage). The engine should be very happy but I don't know how long the starter will last.