The function of the 150 ft. of RG-213 is to accomplish the appropriate impedance rotation that will allow the antenna to be tuned with your wide range tuner on all bands, 160 through 10 meters. For example, if the system is installed with only 75 ft. of LMR-400, you would not be able to tune 160 or 80 meters.
Plan to install either 150 ft. of RG-213 or 185 ft. of 400MAX coax.