The way to make your v6 the loudest is to get an off road midpipe and straight thru mufflers...BUT this will not sound the best, in my opinion.
If you are going for quality of sound, or the best tone, it's really going to be a personal preference. But I will try and guide you by suggesting you look into chambered mufflers...Flowmaster, Pypes, certain Cherry Bombs, etc.
Most straight thru mufflers like Magnaflow and Classic Cherry Bombs are going to be louder and even flow better than chambered mufflers, but won't have the tone quality.
Borlas are straight thru design, but somehow don't sound too bad on v6's.
Stick with some sort of catalytic converter, whether it's the stock ones or hi flows on an aftermarket h-pipe. They will keep the v6 mellow, which is what you want.
As said, replacing the stock y-pipe is a good idea as it is restrictive and shoots down any possibility of your stang having "true" duals. You can purchase a midpipe, either x or h, catted or off road (no cats), or have a shop weld divorced pipes in place of the y-pipe. Doing this will improve sound and flow to a degree.
Installing a cam is not illegal. Usually most v6 owners will upgrade their heads, intakes, and cams at the same time to utilize teardown time and gain more power, but some v6 vendors do sell cams specifically made for that lopey sound at idle. That will definitely improve the sound of the v6