What kind of pipe is on either side of the backflow? If it's cast iron, you're going to have a hell of a time cutting it with a sawzall. Regular metal blades wouldn't scratch it. You can get ceramic blades, but it's so slow-going your arm will fall off. A circular saw with a masonry blade on it works well, as long as you can get most of the way around it, the sawzall could do the rest. Your best bet would be a snap cutter, maybe you could rent one somewhere.

Of course, if it's PVC/ABS, none of that applies.