Calculate Voltage Drop

From the information given:
Voltage drop; Less than 24 volts.
Less voltage drop; Bigger cable.

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Ohm's law
Not just a good idea;
It's the LAW!

No ampacity problem with 6 A per any conductor [from 14 awg copper and 12 awg aluminum]
according to NEC art.310.
Using a single conductor per + and a single per - for 3% voltage drop of 24 V [=0.72 V]
[Voltage drop in d.c. system it is V= Resistance*I[current]. So, you need a conductor of a resistance of 0.72V/6A=0.12 ohm for 2*300 ft. That means 1000/600*0.12=0.2 ohm/1000ft.
Let's say 60oC [maximum conductor temperature the temperature factor is 1.16 ]
kT=(230+60)/(230+20)=1.16
We chose 2 awg copper conductor 0.156 ohm/1000 ft at 20oC and 0.156*1.16=0.18 ohm/1000 ft or 1/0 aluminum 0.160 /0.185 ohm/1000 ft.