Do you have files with a cutting edge and a safe edge so you remove metal from one surface only? I bought one a long time ago for filling rear sights. I can open up the width of the notch without messing up the depth.
My only advice is to be careful and take it slow. It's a lot easier to remove more metal than it is to put it back on. And when the lugs fit, you can get a variety of different length barrel links to get a tight, consistent lockup. But I bought a Dwyer Group Gripper instead. I saw a hand tool for scraping away metal between the lugs in a slide a long time ago. A carbide blade maybe, but it was way too long ago to remember. That's all beyond my area of expertise.