A tip for voltage leveling between 3.3V and 5V devices, use 74HC4050 Hex Buffers.
They're cheap considering that 1 chip levels 6 lines of 5V down to 3.3V in a small package that could be built into a patch cable. An SPI bus only needs 3 lines leveled, would RPI ever run 2 SPI channels to 5V devices? One end on the RPi and two ends for 5V SPI?