Your comment about the two shortest 747 routes being at least 600 miles was a give away to one of the answers: DL DTW-ATL, which is 602 miles long. DL operated DTW's first 747 service in Dec 1970, on the DTW-ATL and DTW-MIA routes.
My guess for the other 747 route would be DL ATL-DAL, which is slightly longer. I think DL's first 747 service in Oct 1970 was ATL-DAL-LAX.
The other airline flying DTW-CLE might be Northeast, on a DTW-CLE-BTV or MHT route, with DC-9 equipment.