What motor for a Vortec SC?

I just rec Finished my solid roller 383 setup and I was disappointed. I was wondering what type motor would be best for a 6 PSI Vortec S trim? I didnt know if I should get a stock motor and slap some new pistions in it? Or do a 355 or a 383? The 383 I have now has flat toped pistions, Stock rods, and a nodular Iron crank. If I change out the pistions will that block/combo hold the PSI? I know I will need a diff cam. But my questions is what would you build? A stocker, 355, or 383? and how would you go about it?

Thanks
Matt