That's not a problem for the heads. The problem is if you try to use the stock head gaskets. It won't take that much continuous pressure over the long haul and you will blow the big white fart out the tailpipe eventually.
Get an appropriate headgasket set. The metal ones are fantastic - especially if you use ARP studs or bolts too. Since head gaskets are not reusable make this a long term committment - rather expensive for a temporary deal. Consider shaving some 1/32" off the heads to level them but make sure the valves don't hit with clay as you are changing the pistons to get to this number initially. There is lift and there is duration on the cam and then there is timing the cam so things don't touch. Considered roller rockers?
Building a motor piece meal is never going to save you any money BUT you will learn a great deal in the process which is invaluable down the road as you continually redo the thing. There is power and then there is reliability and seems like the two never make friends.