I would expect that a 318 to reach 500 hp (comfortable number to run tens) would need lots of compression (race gas?), big roller cam (not cheap, not so streetable) GREAT heads, (not cheap) and plenty of carb, and maybe a tunnel ram.
I think a cast crank 408 would not need the expensive version of those parts, nor the street unsuitability. Just a much nicer street combo, for (I think) less money than a 500hp 318.
