Remember, that the World Championships did not begin until 1983. I have a feeling we may be talking about Bill Rodgers, multiple World Marathon Champion, since Boston was pretty much the "World Championship" back in the late 70's and early 80's. Rodgers has good arguments for 1979, as he was ranked #1 in the world.
Porter was absolutely brilliant as well, but Bill Rodgers does have a bronze medal from World Cross. Rodgers was ranked 6-times (1-6-1-2-1-7) in the World for the Marathon, whereas Porter never had a world ranking from 5000, 10,000m or Marathon.
What's great about both these guys is longevity. Do you realize that Pat Porter was ranked in the US, over 10,000M from 1982 all the way to 1996!!! but never # 1 and once over 5K. Bill Rodgers, 1975-1986 with rankings, including twice over 10K.
Rodgers dominated his era, whereas Porter was one of the best in his, but I will support the sentiment of most on this topic: Bill Rodgers was a Marathoner/Road Racer, whereas Pat Porter was a Cross Country Specialist.