Something to consider. The 400 with the smaller main journals means that there is more metal in the main webs on a 400 than there is in a 455 = a stronger block. (It does matter which specific 400 you have, some of the later models are actually weaker). If you're building for serious power, or think you might want to "some day", then a strong 400 is the better choice. They also oil better than the large journal engines (smaller journal = less rotational force working against the oiling system.)
When I was deciding on my engine build, I had two routes I could go. I had the original numbers-matching 400, and I had a 1970 4-bolt 455 block. I built my 400 as a 461 stroker, and I'm very happy with the results.