Don't think I'd ever would build a 500 hp 318, The only ride I could think of putting one behind if I was gonna build a gutted corner carver with a six speed and at least 4.56 gears, but I'd probably hold out for a 340 block still. To me once you start going over 425-450 hp I'd start thinking Big Block.
To me power ranges
/6 150-250hp
273 200-350hp
318 250-400hp
340/360 300-450hp
408/416 400-550hp
383/400 400-550hp
440-499 450-650hp
500 plus 600 plus hp