eh at least 50 thou or so. put it all together and clay the motor. or use a piece of solder. i like the clay better myself but eh whatever. as long as there is bit of clearence you should be good. just remember high rpms do crazy shit, stretch rods etc. better too much than not enough. also make sure the valves dont hit each other from wild cam gear adjustment. might want to make a note on how far you can sweep each cam before valve to valve contact becomes an issue before you fire it up.

might i ask why you milled teh head 20 thou? was it warped, or you just wanted to get teh compression up? cometic i believe makes gaskets in various thickness to accomidate for the head being cut so much. also remeber the cam timing marks arent going to line up quite right and the cam timing will be off a bit from the distance change from crankshaft to camshaft due to the milling. so you might want to get out out the degree wheel and dial your cams in.