You'll find that planing an endgrain slab (cookie) with a conventional planer will result in a really poor result, if not total destruction of the slab and maybe the machine as well. A widebelt sander with a 36 grit belt does a nice job, but leaves heavy sanding scratches that require lotsa further sanding with 60 and 120 at the very least. I've flattened slabs like yours that way a few times and it averages about two hours. At 85/hr (what most shops charge for widebelt time) that gets a bit expensive. A better, cheaper solution is to make your own router jointer. It's basically a set of tracks, a cairage that travels back and forth on the tracks, and is adjustable up and down, and a router mounted to a plate that can travel side to side in the cairrage. With a 3 hp router you can take off up to a 1/4" a pass. I run mine with a 21/4" fly cutter. I've used this method pretty much exclusivley these days. Once you've got the two sides flat and parallel to eachother, it's a matter of 15 minutes or less on the widebelt to sand out the tool marks.