650 NA plus 7 psi is about 975hp. Blow through carb is good if you’re going to run non intercooled. I wouldn’t worry too much about the block. They can take a lot of power when prepped correctly.
@Turbo440Dart just made more than 800 on a stock block and we were hardly leaning on it, and his doesn’t make nearly that much NA. You can use smaller turbos but the way they’re sized you start to run in to back pressure issues with the small hot side. I use twin 67/62s on my W2 magnum at 360 inches. I bet those would spool instantly on 512 inches and backpressure would probably be up near 2:1 at full song with 650na hp. Twin 70s with .96 hot sides would be about as small as I’d recommend on that big of an engine.