This tread is 100 years old. If you go through a lot fo treads you will find all answers on you any raised question.
On this question I will answer for you-do not cut bottom edge of exhaust windows. If you cut and it will open while piston in Top Dead Center your engine will have decompression of crankcase, it will mess up all your timings, millage and power. What is the reason you want to cut it> do you think if you make smoother pass for better flow it will run better? It doesn't work that way, it is not sewer pipe with shit flowing in it, there are many processes involved, including sound wave, so that "smoothing" will do nothing in terms of better flow. But decompression will kill your engine performance, specially with tuned pipe. But I can guarranty you, it will work much better without tuned pipe.