Those Vortec heads typically are cast with 66-68cc combustion chambers. You are very unlikely to get a set with a true 64cc size. You will need to verify this with modeling clay or a burette. Assuming you go with those 20.40cc pistons, that mean your static CR is going to be 9.1-9.2:1. You could send the heads to a machine shop and they can deck them a little to reduce chamber size, but it is probably easier and just as cheap to get some different pistons from the get go. The 18cc pistons in the kit as suggested by my87Z would get to you to 9.3-9.4:1. Are you trying to run on 87 octane? With a nice tight quench from that zero deck and .040" head gasket, you can get more compression with less timing advance and detonation (thus be able to use lower octane).
I think you should probably bump that CR up a little bit to match the cam, and also more compression = more power: http://www.crankshaftcoalition.com/w..._compatibility
In my estimation with your setup on that low 9ish:1 compression you should be knocking on the door at 400HP. I have a desktop dyno and will plug the numbers in later and send you a PM.
A roller cam and roller rockers would help achieve your power goals. But the cost of roller cam, retro-roller lifters, correct length pushrods aint cheap! And do you really want to top off that roller setup with factory stamped steel rockers? Heck no!