The manual has the way to set the belt tension. They actually have you measure the length of the spring & if I remember right it is around 7", but I would look at the diagram to see the points they want you to measure from.
My friends 61 kicked the belt off when it was new also, but hasn't had it happen since. The belt is at a pretty good angle with the deck in transport position, so I'm not sure if that could contribute to that when you may hit a good bump or not. His belt had a twist in it that may have been the cause.
As for running out of gas, when they empty a tank it takes some choke & cranking to get started. Must suck the line dry!!
I never have to use choke to get mine started except in the cold winter months when it sits & I start it once in a while. The briggs has been the easiest starting motor I have had compared to any Kohler or Kawasaki.
The suspension seat I put on mine did raise me up a few inches. It feels different at first, but you get use to it. I did notice that it affected "floating the deck". Put my leg at an awkward angle & made it where my knee and drive sticks would hit sometimes. I never adjusted the sticks. I put the factory seat back on just to do a comparison. I demoed the Gravely with the air ride seat and it was a nice seat, but the Ferris still rides better.