You've probably heard before that two eyewitnesses have come forward and said that they saw George Romney and MLK march together. Well, now FOUR historical books have confirmed the same thing. Also, Mitt's brother, Scott states that he heard his father say that he had marched with MLK (in that same last link).
My scientific background also reminds me that positive evidence (like the two lady eyewitnesses' claims, or the 4 books cited) is always stronger than negative evidence ("I didn't see them together" or "newspaper columns of the time failed to confirm").
It is not out of the realm of possibility that, despite all this evidence to the contrary, they did not march in the same parade. But with historical books and records getting it wrong it's hard to claim that Romney "fabricated this out of whole cloth" like opponents are implying. I think most rational people are willing to not get caught up in the figurative use of the word "saw" when we can all "SEE" that George Romney was truly a champion of civil rights (the silver lining in this whole affair).
Regarding the 1978 quote that apparently Romney said that he marched with his dad and MLK through the streets of Detroit . . . well, given the apparent lack of journalistic accuracy in this whole affair I'm afraid that I wouldn't be surprised if it was a complete misquoting of Romney. However, if he did in fact say that, then at least he hasn't propagated or inflated that error in the intervening three decades (something that serial liars/exaggerators cannot help but do . . . the stories usually get better and better with time!!).
Update . . . A Harper's Magazine article from 1967 stated that George Romney and MLK marched together. You need a subscription to read the PDF. Here's the search results which gives some of the text and the relevant quote is:
"When the Reverend Dr. Martin Luther King marched in Detroit three years ago, Romney marched with him. He is proud that he helped write a state constitution that has the most comprehensive civil-rights guarantees in the nation, including open occupancy in housing."