I don't really get the cost and hardness factor, most 408 builds are stock block and aftermarket everything else and a similar 360 build might use stock crank too, other than possible piston cost,
What so difficult and or more expensive for the parts for a 318 build ?
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.
