is this a stuoid exhast idea?

your v6 will never sound good. the best option for your car is to leave it alone. If you do want it louder, I suggest checking out youtube and doing a search for v6 mustang exhaust. Most videos will tell you what they have for exhaust, and you will be able to hear them as well.

Cutting off one muffler of the two is not a good idea, due to the fact that if it is dual exhaust with out a cross over, half of your exhaust will sound like crap, and the other half will still sound factory. I.E. take 2 of the same car put them next to each other and chop the exhaust off of one, what are you going to hear? The car with the loud exhaust will drown out the sound of the quite car. If your car does have a cross over pipe and you remove one muffler, the majority of the exhaust will escape out the side without the muffler, because there would be less resistance, and that would be the easiest way for it to flow. If you have two mufflers in line ( -----===-----===---- ) looking kind of like that <---, removing one will have very little impact.

sorry, but the only way to make a V6 sound like a V8 is to add 2 more cylinders

Yes it's a stupid idea.