8x8x8 multiplexed LED cube with an Arduino Mega 2560

Seems to be a logic level part

Will have at least 1V drop across it, see Figure 2.

TPIC6B595 will have 3-4 ohms of resistance, see Figure 7, at 20mA = another 0.07V

So as long as your LEDs have Vf <(5-1-.07) = 3.93V you should be okay.
Adust your current limit resistors accordingly.