SPI
.. is a quite trivial matter. It is a part of 019 now, isn't it? Here a library
http://www.arduino.cc/playground/Code/Spi
NSS
.. can be simplified a little bit, especially when you constrain to a few bit-rates only. It can be re-written using USI. The size of the receive buffer must most likely also be reduced.
I have ordered some 84 chips - I cannot start testing before next week.