It sounds like you are talking about synchronous communication - no start or stop bits. If that is the case, you can use the SPI function of the processor. For a 16MHz processor, you would set the baud rate register to 413 to get the baud rate you are looking for.Read the Atmel328 data sheet topic about the USART in SPI mode.
It looks like the SPI library will do what you are looking for but you'll have to modify the source for your custom baud rate.