I think you are confusing the RS-232 Serial port with the SPI port. Serial.begin(300) sets the RS-232 serial port. It has nothing to do with the speed of the SPI port. I really doubt that you can get it as low as 300 baud.
The SPI speed is set by specifying a divider like this:
SPI.setClockDivider(SPI_CLOCK_DIV2);
[edited]This can go from 2 down to 128 in powers of 2 - for example, SPI_CLOCK_DIV32
IIRC the default is SPI_CLOCK_DIV4
Why do you want to slow down the SPI clock?
Pete