I'd do it with a router with a 3/4 bit that will make a flat cut on the bottom. Clamp the straight edge in place so it doesn't move between passes. With a full sized router, I would make three passes, each an eighth inch deeper. Mark the top of the router base with an arrow and keep that running against the straight edge to make sure the router doesn't rotate and possibly make a slightly curved cut. If the slot is just too tight, you can use some sandpaper wrapped around a stick to widen it just a bit. Probably a good idea to make a test cut in some scrap and see if your track fits. My own preference would be to use a mortising bit that makes a nice smooth and flat bottom.