Maybe the underside will give an indication of how much material you can remove. The shape of the underside may not be what you expect based on looking at the top, especially after removing some of the material, so let the underside help guide your decisions. I wouldn't be too concerned about the existing bar mount holes, whether you plan to weld them up or not. Looks like you have the bulk of material taken off. I like a die grinder and a double cut rotary file for contouring things like this. Hand files are about as fast, and make straighter surfaces, but are sometimes a bit hard to get into some of the inside curves. Put wax (or bar soap if you don't have any wax) on the cutter and it won't clog with aluminum. Any kind of grinding stones are usually pretty tough for aluminum due to clogging.