The Vortecs are 64cc chambers, and you need the deck height (do you plan to zero-deck?) and the head gasket compressed thickness. Have you made the decision for hypereutectic or forged pistons? Forged is required if you're going to use any power adders (NOS, blower, etc.) Finally, I'm assuming this is a 5.7" rod 355 (.030 overbore)
Typically, a flat-top with two sets of valve reliefs (for installation on either bank) will have about a -5cc volume, which provides around 9.8 CR with a 1.560 compression height and an .025 deck. You need something with around a -11 dish - a Speed Pro H670CP30 or something comperable.
Best bet is to talk to your machine shop - chances are they have preferences for the pistons they work with and can get 'em at about the same price you'd pay somewhere else.
NOTE that this is not a recommendation - as you didn't provide all the information needed
Translation: if you buy these and they don't work it's your fault and not mine