Why are 318's low HP and torque?

My 1965 has a stroked 273 CI using a kit from Eagle. The 4340 forged crank has a 4" stroke like a 360 CI. With a .030 overbore this is 331 CI. With a good solid-lifter cam, some head-work, pistons at about 10:1 compression, single plane intake & four barrel this combo makes over 400 HP easy. I am even using the original cast iron exhaust manifolds extrude-honed becuase I didn't want headers on it. You can do the same thing with a 318 and get 390 CI.