Datasheet: Fig 19 shows a 12 volt to 5 volt 5 amp design. Add R1 to move up to 6 volts.
Vin 10 to 36 volts (will pull under 3 amps from battery)
Vout 5 (or 6 volts) @ 5 amps
Your goal is too modest. You are asking how to convert something that can deliver 144 watt-hours to something that only delivers 27 watt-hours, so you are asking for something very inefficient. This can easily be done with a series voltage regulator rated at 5 amps and with a big heatsink. Consider the LM338K mounted on a heatsink that can deliver less than 1 degree C per watt of thermal convection.
A better goal might be to ask for 6V but with 20 A-H capacity.