Voltage divider arduino 5v and Ethernet board 3.3
From experience with 3.3v ESP8266es with 5v Arduinos, only the signals to the 3.3v device need to be converted (5v high -> 3.3v high). A 3.3v level to the Arduino is high enough for 5v logic to still read it as HIGH. That means you can get away with just a voltage divider on each of the Arduino -> ENC28J60 pins.