In a mild to medium build motor, I am a believer that there is no substitute for cubic inches. Especially in a street motor. Just as Steve said why give up 50 to 70+ cubic inches. The 400 also has lots of torque which means you can run a higher gear and still have plenty of seat of the pants power at lower RPM, which is where you need it on the street. Regarding the external balance, that is not really a big problem, 454s are externally balanced too. You just need the right flywheel and balancer.

Good luck what ever you chose

Pat