get headers, make the downpipe the same size as the header collector, and at some point make the step down to 2.5" and run whatever else you want behind it (resonator, mufflers, glasspacks, etc).

there is NO reason to run 3" unless you have a turbo car. it will just be loud and youll gain no power. true dual is...well it's stupid. it is nearly impossible to run the piping and youll also gain no sound or performance from doing it. keep the exhaust similar to how it is now.