As stated, don't file the dovetail slots.
Take a little bit at a time off the base of the sight itself.
File, fit, repeat as necessary.
You don't want to file off too much though. You'll want to be able to tap the sight into the dovetail with some force. Too loose isn't good either.
When I built my squirrel gun, I mistakenly filed the dovetail in the barrel a little too big. I tossed the sight in the parts box and bought an oversized sight and just filed it down to fit the dovetail.