The pistons have the same bore I believe. Im at work so I can't double check that. The only thing that might be different is the dome size on the pistons, stock. I would just go with Y8 pistons as they are interchangable. The valve reliefs might be different on the Y8 to allow for higher lift of the valves, again, Im not remembering the lift/duration for stock Y7/Y8 cam profiles. IIRC you're goin forged internals, so just go with the Y8 pistons and everything should be fine. If you want check out google for the piston profiles, check my earlier posts for D series build ups and tech posts, I've posted a bunch of piston information.

When you're building this motor, remember to use a VTEC oil pump (y so it'll help the engine rev higher. Im not positive on the crank in the y7 either, I dont know how high it likes to rev, so be careful with that one, do some research. If anything you can get the rotating assembly balanced and everything will be fine.

Hope that helps man.