Sacrilege - gearing up a fixed

hijacking the topic, it's perfectly fine to put a 5 speed block on an 130mm rear spacing? I'm building a bike up for my dad, and decided to have 1 chainring and 5 speed rear as he never change gear on the front.

I'd imaging it should be ok, if you're using an existing wheel you'll probably need spacers and have to re-dish the wheel.

As Alc says though, you'll need a hub that'll take a screw on block rather than a modern freehub