The largest margin of victory in the Tour of Flanders' modern era is Eddy Merckx's 5'36" win over Felice Gimondi in 1969. Marino Basso, third at 8'28", represents the most distant finisher for the final podium place as well.
On the flipside, since 1986 there have been 10 finishes where the top two (or more) finishers were given the same time. And that's not including two more finishes with gaps of 1-2". Of the same-time finishes, three of them featured Leif Hoste finishing second... tying a record.
As ignominious as Hoste's exploits sound, two other riders own three runner-ups: Sean Kelly and Johan Museeuw. Pretty good company. Museeuw got his wins eventually; Kelly never won de Ronde.
Info courtesy of Bikeraceinfo.com.