How Much To Mill GT40 Heads? .030"?

Hello, since i built my 351w, i've had belt problems(finding the correct size) and one time on the highway it flung off without my knowing, it was driven for about 3-5 minutes until i noticed the temp guage was maxed out(stock guages). I immediately pulled the car over and it was boiling out the overflow tank....So now i ordered 1011-2 head gaskets and some 1250 intake gaskets. I will be tearing it down today after work and need to get the heads milled, my question is how much? i'm thinking .020"-.030" to bump up my compression while i'm at it. I have 9.5:1 compression pistons(with a 63-64cc head), and my cam has 498 intake and 510 exhaust lift, i'll try to find a link for ya...should i have any clearance problems? the head gaskets final squish is .039" and is 8.5cc, Thanks
EDIT: i am running scorpion 1.6 RR's, and this cam: http://northernautoparts.com/ProductModelDetail.cfm?ProductModelId=1696

~Mark~

thanks guys i think i'm gonna pick up some clay anyway just to be sure, the last thing i want is bent valves. The pistons have 4 deep valve reliefs(overkill IMO) so i'm not too worried anyway, i pulled the heads tonight and cleaned everything up for the machine shop tomorrow, hopefully they're flat but i dont expect it, either way i'm gonna ask for .040" off to bump my compression up. Also while tearing it down i rmembered i only torqued the head bolts to 75 ft lbs and didnt even use liquid teflon on the lower head bolts! i'm sure neither of those things helped..so wonder it started leaking before 1K miles

~Mark~