Valid options for output port on Mega 2560
It would seem this library needs a single pin to communicate. So it
doesn't really matter whether the port you choose has only a few pins
available. Also, you do not care that some pins can have extra
functionality, like SPI, I2C, etc, unless you do plan to use that
functionality.
There is one think you should be aware of: the library uses the in and
out assembly instructions to access the port. These instructions can
only access the ports that are mapped in the regular I/O space of the
microcontroller, not those that are in the “extended I/O” space. In the
case of the ATmega2560, this means you can use any of the ports A
through G, but you cannot use H, J, K nor L.