The only “up side” to starting with a 318 is whatever the price difference is between a core 318 short block and a core 360 short block.
And if you can’t find an off the shelf piston for the 318 that fulfills someone’s particular requirements, and have to get customs........ you’ll be starting out with a price deficit(because it’s not a problem at all finding off the shelf 360 pistons for that build).
I would say if your looking to build 500 hp with any v8 by any make or size it's gonna be around 280-320 ish cfm heads, 235-245 ish cam with .500-.600 ish lift, 10-12.5 ish cr and 750-950 carb plus the appropriate intake and headers etc...