Either conventional hex head bolts or flange head hex bolts... The flange head have a round flare at their base providing a little more distribution of the pressure. These aren't supposed to provide a lot of pressure anyway and just about anything will do if washers are used.
BTW - If you stripped the philips you're either not using the right size screw driver (it's a #3, IIRC) or you're tightening them way too much. Tighten them the 12 in lbs they're spec'd for. If they leak after that then replace the seals and make sure you didn't forget the RTV at the #1 cam caps. Again, that's 12 INCH lbs, NOT FOOT lbs!
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