The 350 has a smaller bore than the 400, restricting the size of valves that you can use. They both have the same stroke, which means that they will turn about the same rpm, so the 400 will make more power because of better flow and more cubes. Like Jim says, easier with a 400.

The Edelbrock package on a 400 will make around 430-450 hp on the dyno if everything else is done right.