26 June - 19 July 1958
The following information about the Tour de France 1958 is taken from Wikipedia.org.
The 1958 Tour de France was the 45th Tour de France, taking place June 26 to July 19, 1958. The total race distance was 24 stages over 4,319 km, at an average speed of 36.919 km/h.
Only two stages in Belgium. It however started at Brussels on 26 June 1958 and in Ghent the day after.
The stage on 26 June had a length of 184km, ended in Ghent and the winner was André Darrigade (FRA). The stage of Ghent went to Dunkirke France, was 198km long and the winner was Gerrit Voorting (NED).
The below philatelic material is issued J. Forret and had a cover and card for each stage (totalling 25 sets). I have only the cards of both stages, and the cover of the first day.
The only relation of the Tour de France cards and covers to the Expo 58 is that the stage of 26 June effectively started at Expo 58 - no more information available
The cover of the first stage is quite harder to be found: