Even if you keep the rest of those slats rip out the wacky corner and the bottom couple inches. To match the right size of slats just get the closest plywood sheet and rip it into 3 inch sheets. Plywood is magical furring strips: any thickness you want, flat, cheap, never splits. Any wall you are completely ripping out the slats to reinsulate, use furring strips. Old walls may look straight but they are gay as americas politicians. Even if the insulation looks okay, replace everything. Even if it doesn’t get wet fiberglass has a half-life of 30 years. There is one magical trick you can do that will triple that walls insulation for a couple inches of thickness. Get xps or eps, whatever is cheaper in your area. 1 inch or 2. Then just nail that into your wall and mark your studs and electrical so you don’t hit it on the other side. Spray foam with just the can or gun all the seams and corners. You will need to get outlet extenders of course. Don’t do this if the idiot ran plumbing in the wall! It will freeze, this insulated incredibly well. Then use 3 inch drywall screws to attach your drywall to the studs, through the xps. Your walls will be straight because the styrofoam will eat all the unevenness. The insulation value is unparalleled.