Whats the Model Number of the mower?
I assume it is a geardrive? If so, the drive belt just went around the engine pulley, trans pulley and a backside idler pulley. The backside idler is the one that slides to adjust tension and is up under the rear right corner of the unit.
The belt should have the same tension in 1st gear or in 5th, 6th or 7th depending on model.
Unless it is a variable drive set up?