I want to run it at 6 MHz.
Sorry you can't. If you look at the data sheet you will see that the clock frequency is derived from the crystal divided by either 2, 4, 8, 16, 32, 64, or 128.
As the arduino has a 16MHz clock you can't get 6MHz. Closest you could get is if you select a divider value of divide by 4, this will give you 4MHz.
This is controlled by the two least significant bits of the SPCR register and the least significant bit of the SPSR register. See the processor data sheet for more details.