In the 80's yes, they're maybe close, but I would still give the edge to Fleche in that period. Fleche Wallonne was included in Desgrange Colombo right from the start (3 years before Liege), and since then always been a part of premium cycling competitions, until beginning of the World Cup in 1989. Gent on the other hand, although had great roll of honor, wasn't included in Super Prestige Pernod until 1980. Later, same as Fleche, wasn't part of the World Cup, but from 2005 both races were in the World Tour, and I would say that today they're pretty equal in prestige and significance.I will have to check but your description of the 8 top races doesn’t fit what I’m remember going back to the ‘80s. Gent was just as big or bigger race than Flèche, and Championships of Zurich were also close to that level.
Zurich was mainly domestic affair up until mid 60's, with some flashes right after the WWII when Bartali and Coppi attended. In the 70's race was held mid-week in May after Frankfurt, and gained some quality field. From 1979 race was included in SPP, and then it's significance rose, especially after date was moved to mid August in 1988 and from next year was part of the World Cup. In that period you could say that Zurich was bigger race then both Fleche and Gent.