A 400M is a torque monster, I used to own a '78 F250 that had one; it lasted over 200,000 miles.
Those engines had some of the most restrictive intake and exhaust manifolds I have ever seen. Find yourself a nice 4bbl intake and carb, and a cheap set of headers. The stock cam is very mild, made for low end torque; you could grab some easy hp by swaping that for something w/ a little more duration. Look around on ebay, parts are easy to come by. Intake and exhaust parts are interchangeable for the 351M and the 400M, so keep that in mind when you're shopping around.