I would use 6 TPIC6B595 latch/drivers, you only need 3 pins of the Arduino , and it will give you 48 high power 500mA sink outputs to switch your relays ( it has the 50 volt voltage clamp diodes built in )
The relays will give you the isolation you need.
I use this arangement for LED displays with up to 20 7 seg numbers.