I see enough material in the pic of the pipe and stove to make your wrap around. Those blue banding straps will work as a wrap around. Or you can take the lumber from the top of the new stove and nail them into a t square and lay it with the T hanging off the edge of the square end of the pipe and just slide it around the pipe while marking off the other end. An abrasive wheel on a skill saw can be used to make the cut, just follow your line. You might have to take your grinder with a cutoff wheel to get the cut started, but once you have a slot big enough for the saw blade to drop in, it will cut pretty fast. It will get plenty hot as you are sawing too, so it wont hurt to take a few breaks while you doing this. You will probably need the break time before your saw does