It depends on what horsepower level you are building. What Intake manifolds and what headers will fit your car, as what heads you are going to put on it.

First the 400 is a little stronger, and the rotating weight will be slightly lower, so for a all out engine it is the best choice.

If you want to run a crossram for example, you might find intakes are hard to come up with.

If it has a head with a raised exhaust port, 440 and 400 blocks have a little difference height, couple that with aftermarket cylinder head heights and you may need custom headers.

Also any HP level over 650 HP stock blocks can be a little weak, aftermarket blocks are hard to get right now, but a 440 style aftermarket will be easier to come up with.