Here is a thread all about a build up on this engine.

351M/400 Performance Build Up

How We Got Over 500hp from Ford's 400M Engine - Hot Rod Magazine

And Tim or Tom Meyers I think that is his name has a machine shop that specializes in this engine and has some good stuff there too.

http://www.tmeyerinc.com/

But yes, this engine can be built up huge and is a good choice for towing in lue of an engine swap.