They're 1/4-20, I just stuck a bolt in an old head to be sure. The length will depend on how thick your pan and valve covers are plus your gasket, aluminum ones are quite a bit thicker than steel. They aren't a high stress item so as long as you have at least 1/4" or so of engagement you should be okay.
Lots of times the fastener kits seem kind of expensive, but once you add up how much you spent at the hardware store, you end up spending more. My local hardware store charges way too much unless you buy a box of 100.
Check with summit, they have bolt kits that aren't too bad I think. Make sure to order all your other bolts you may need (like intake, water pump, etc) at the same time and it'll save you money or at least a few trips to the hardware store, plus the bolt kits usually look pretty good too.