The main difference is the stroke of the cranks. This is what makes the 400 standout from the other engines in its class-- it has a 4" throw, vs. the 3.5" throw of 351M. You might start by doing some background reading on the differences between the 351M and the 400.

You might try

http://www.clubfte.com/users/mil1ion...atedLinks.html and
http://home.earthlink.net/~bubbaf250/

These links are to sites where someone put alot of time and care to the research of this engine family. It will give you alot of good history and a background to the engine etc. , plus practical guide to rebuilding stock or custom. Also do a search on the 351M buildup here. Parts are somewhat hard to procur, especially for custom rebuilds. This engine isn't a Ch#$vy 350 after all, but there's a couple people here that will help a whole lot with availability and i would recommend using them in favor of the route I took. They've got good prices too, but ordering takes a little preplanning. Come back here and repost for a good recipe after you've narrowed down the purpose / intended use of the engine.... mine was torque favoring truck for pulling, for example. Each build is different based on this.