New Gas Tank Sending Unit Leaks

There isn't a good way to bend any metal and improve the fit.
The goal would be to close the space between the tank wall and the retainer that is welded on the tank. Sure you could capture the gap in channel lock pliers but... If the tank wall bends instead of the retainer, you're screwed. The tank wall and the flange on the sender need to be flat. The retainer and lock ring can be distorted since hey only pinch the gasket between 2 flats.
Best plan is make the new seal ring act as thicker. Use a rounded punch to put some dimples in its flat surfaces. Dimples toward the retainer will likely work better than dimples toward the sender.