Well, you've got a couple options: First, I'd take the sight off and put the hopper back on. Then try to install the sight with the hopper in place (at least then you could play). Also try mounting the sight further back on the dovetail to minimize interference with the hopper.
For the future, an offset mount wouldn't be a bad plan - the Lapco model is pretty deluxe, while JCS makes a more "bare bones" model that's cheaper.
You could also leave your hopper off and replace it with a 30 rd tac cap (they're nothing more than the black ring and cap off the top of a 140 rd guppy), if you don't shoot a ton of paint at once. Then the hopper doesn't interfere with your line of sight either.