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.
The Arduino hardware SPI is limited to a fixed set of frequencies derived from the processor clock divided by powers of two. Successive bytes may not be bit-synchronous with each other - you'd have to test that.
To generate an accurate fixed frequency the use of timer1 (which has 16bit precision) is a possible way - using a timer interrupt would be the most accurate way to use that timer to push bits out.
However the UART is probably the simplest way (if you can configure 0 start bits and 0 stop bits?)