As near as I can tell, using your pictures, your sight dovetail is filed out of square with the long axis of your top barrel flat.
As is, this causes the sight to sit cockeyed. Looking at the top of the barrel put a square along the side flat and check your slot. You'll find it slightly askew, mark it. Carefully file the slot square, go slow, check your work often and only file on the side needing to come square. Once you have your slot squared up try your sight in it. It will be loose, the front or back more square than the other side. Take your brass sight, anneal it, and peen the bottom of it to widen it just a tad, light blows will do. You should be able to peen it enough for a firm fit in the slot. If not...
...consider soldering a thin brass shim on the bottom of the sight flat. From there you can dress the sight base to fit your slot. Or, make a sight, they are not hard to do, just tedious, and you could use the sight blade you have and fabricate a new brass (steel, copper, half round-bar) base.
I wouldn't be taking a pair of pliers to that sight, it won't bend, it will shear off.
dave