How can you get more horsepower out of a ford 400?

I have a 1978 ford f-150 with a professionally rebuilt 400 in it, and I was wondering how I could upgrade it to get more power.How can you get more horsepower out of a ford 400?You probably have a 400 M engine, a kind of Modified 351Cleveland that was used for heavy duty usage, like police cars %26amp; taxis. Had one in a '78 LTD II station wagon. The engine didn't stay in production long, only like 3 or 4 years, because the 351 Windsor came out %26amp; replaced it. Because of it's short life, there never was too much aftermarket activity for it. The limiting factor is the breathing thru the heads, so having them ported has to be the first step.