6 feet is about what you need anyway, so the cutting isn't required. If I remember correctly (too lazy to go back and re-read the thread) you need to mount about 6 feet of mast to your chimney to be about 5 feet above the crest of the roof. So, one mount just below the chimney lip at the top and an angle under it with the other mount down at the bottom of the mast with the other end of the angle under it. Four, 6 foot angles, one for each corner comes out right.
Also, if you can manage the $0.70 additional cost per angle it would be worth getting the 1/8" vs. the 1/16" thickness. 1/16" thickness might not be enough when you start tightening down the straps.
Dan