ironspider, I think multiplexing is not ideal if you need to turn on 75 LEDs at the same time. Instead, look for shift registers. The 74HC595 is very popular for Arduino:
http://bildr.org/2011/08/74hc595-breakout-arduino
And you better use an external power supply for powering all your LEDs, the Arduino won't be powerful enough and you may damage it...
Good luck 