Max HP debate

Too many variables and not enough information.
What year block? Early 360 castings used 340 water jacket cores, which gave them thicker cylinder walls... there's a thread on here where a member weighed many different years bare blocks, and iirc they varied by almost 40 lbs. I'm looking for the thread...
Are you going to fill the block, partial fill, or nothing?
Are you stroking it or running stock stroke? What is your projected rpm that you want to run to get your 600+ hp?
I don't think you'll find anyone who says you're not pushing a stock block at over 600 horse, especially since you know at some point you're going to want MORE...! Can it be done? Yes. It's done regularly, with good prep. Will it live? Maybe. My grandpa smoked cigars into his 90s. That doesn't make it smart, but it worked for him. You're not in the 600hp powerband all the time, either. But rule #1 is to start with a solid foundation. An early block would be preferred- an X or R block is icing on the cake. Then you've stacked the odds in your favor wherever the future takes you, and not worrying if you "crossed the line" with the last upgrade.
I just find it odd that you're willing to drop the coin on W9s and all the attendant hardware, but want to possibly jeopardize your investment with a stock block. But, if the block checks out and you're confident enough in the prep work, go for it.

Edit: I see you scored the W9s by coincidence- good going! Should be a sweet motor.