The 400 is pretty much the same as the 327, same block, more stroke. The 327 is a higher revving engine, but the 400 makes more power at the same rpms because of the extra displacement.
Of course, you can build a 7000rpm 400 if you want, all the parts are available from any catalogue... Good thing to remember about building a big smallblock, it doesn't cost any more than building a smaller smallblock, but gives you the same performance of a big block, at a much more reasonable price. Phew. :o