2000 mustang v6 exhaust. flowmaster 40's?
You can't really make a v6 sound good, it'll be ok with 40's, get a H pipe, 40's are 80 bucks each an H pipe will be about 120-160, then install will be about $100-150. Yes you can keep the stock manifolds (the stangs don't come with headers)
so you're looking at $400 just for dual exhaust for a V6...not worth it.
Trust me, don't get glasspacks, they sound good for about 10K miles on a v8 then they start to sound like a rusted out stock muffler.