First, the pipe you describe is not a quiet pipe, it is an unmuffled pipe. It will be loud. I make carbon fiber pipes, and I am holding a couple in my hand to measure for comparison. It sounds like you have a 40 sized pipe. The diameter at the waist gives it away. Your measurement of the "rear cone", which is the convergence cone by the way, of 1 9/16 confuses me, I would expect 2 1/2 or so. It will be too small for your FX61. You can try it and see what it does. Start with a tuned length of 17", that is from the piston face to the center of the convergence cone down the middle of the header when it turns.
I think you would be better off with a red tuned muffler fron Dub Jett. I use them on 2 FX61 engines, and they add about 1000 rpm.
Scott