I'm not Malmo but, in regards to Frank Shorter, I have my theory as someone who actually watched 4 of his marathon races run on TV in Japan (plus Montreal Olympics).
I think his best marathons, besides Munich Olympics, were Fukuoka 1972 and 1976 Olympic Trial in Oregon. For the former, he was easily sub-2:10 pace and, after securing the win, slowed down quite a bit, with strong head wind as well, and ran "easily" 2:10:30. At the trial, he and Rodgers, figuring they secured the spot, "jogged" the last 6 miles together and finished something like 2:11:40. Had he had a several pace makers like we do in todays marathons or tighter competition or thousands of dollars for the course and world record as a bonus, Shorter could have easily run much faster--perhaps then world best of 2:08:34. But we would never find that out, would we?
Dynamics of distance running, including marathons, has changed so much in the last 20 years that it is absolutely impossible to compare times. I would still consider Shorter as the best marathon runner ever from the US simply because of the way he ran against the bests in the world and beat them in so many occasions.
I have NO doubt, had the race conducted the way it is today, he would have gone around 2:08 on December 3rd, 1972 at Fukuoka.