[A930Rocket]

Since you only need to make the opening 1/4”+ or so wider, and give yourself a little more room, I would be inclined to disassemble one side of the cabinet, not install installed.

Instead of trying to break apart the current glued and nailed joint, I would use your Skil saw to cut the box down as needed on the top/bottom/backsides. Once the side is off, you can dress it up the inside of the side you removed,and reattach it.

For the face frame, with an oscillating saw you could detach it from the box, before you cut the box down.

When you reassemble it, the only place somebody might see anything, is the bottom joint on the inside. A bit of caulk will take care of that.

Just food for thought.