Wes, is this for a bracket car or are you aiming at a certain performance objective or class? What is your realistic budget for this project?
Just asking because there's several ways to skin a cat.
My 355 is real mild and consistent, was reasonable to build, and just what I needed for a lightweight bracket car. It probably makes an honest 540hp (never dynoed) and runs 9.30's @ 143.5-144mph. in my 2350lb. race car. It takes minimal maintenance and is consistent as the sunrise.
To hit the 600+ N/A hp level with a similar mild combo, I'd build a 400-style (large bore) block and use a set of CFE/BMF 18 degree CNC heads. A high compression 18* 377-406 would be a reliable, high power piece that could be built fairly reasonably, be durable and consistent, and not be maxxed out. There would be room to grow later if you wanted.
The reason I said 18* heads is because at the 600hp power level you're looking for from a smallblock, any 23* head that can support that level will probably need an offset rocker system. If you have to buy shaft rockers anyway, there'd be NO reason to stick with 23* heads unless it was because of class rules or something similar.
How's that for a justification of a way to spend YOUR money!