connecting to a transfer switch
For a generator of that size, the primary connection would not be through those convenience outlets, but rather directly to lugs sized for the cables required. If this wasn't the primary use of the generator, and you wanted an easily detachable connection, and 50 amps was sufficient, then you could use the 4-conductor receptacle with a cable.

Your model sounds like the G50. See page 34 of that manual:


  3.13
  Connection Lugs
  
  See Graphic: wc_gr002611
  
  The customer connection lugs (r) are located on left at the bottom of the panel behind a hinged door. The lugs provide connection points for attachment of outside loads.
  A large label like the one shown in section Terminal Connections is attached to the inside of the terminal door. It shows the correct terminal connections for selected voltages.
  Connections to the lugs should be made by running the power cables up under the lug door in the bottom of the panel and into the lug. Use a 3/8 in. Allen wrench to tighten cable connections in place.