Jamie,
Camillo beat me to it. I have the PlanePower 60A alternator and am planning on using 6AWG cable. My 6AWG is from B&C and is M22759/16 rated to 150 deg C. My planning too is also based on AC-43-13-1B.
Table 11-9 in AC-43-13-1B (Copper wire, continuous duty) gives 76Amps max for 6AWG cable rated to 150degC. Note though that this table is actually for cables in groups, harnesses or conduits at 60,000ft! At lower altitudes and with a single wire to better disipate heat, the max amps ought to be even higher. This table gives max amps of 103Amps for 4AWG and 57Amps for 8AWG, again both for 150degC rated wire.
For another datapoint, note that Table 11-3 gives a CB protection value of 80Amps for a 6AWG cable (but with wire spec of MIL-W-5088, which I am not familiar with). This table also says protect 4AWG with 100Amp CB and 8AWG with 50Amp CB.
Finally, regarding voltage drop, 4ft of 6AWG cable carrying 14V/60Amps has a voltage drop of just 0.095volts which is using up about 20% of the suggested max voltage drop of 0.5volts all the way to your endpoint device. Using 4AWG for this 4ft run you'd have a drop of 0.06volts or for 8AWG the drop is approx 0.15volts.
Hope that helps.