Hi. I've got a 1969 Ford Mustang Mach One with a 377 Windsor stroker engine with JBA headers with 2.5 inch inside diameter pipe. I had dual 2 chamber 40 series flowmasters(42541) for one year discovering that they were very loud for street use. I think they are more for track and drag strip use. I started getting complaints from my neighbors and also it was deafening to my ears due to high interior resonance and they're sitting around my house. So I replaced the mufflers with delta flow 50 series 3 chamber flowmasters.
I discovered the exact same thing and changed to the same muffler. The interior resonance of the 40's made me grit my teeth at certain RPM. I would be interested in hearing the new "super 40's", but I would not think that they are more quiet than the 50's.