Upgrades to my stock 273 67 Dart GT

yeah man, it'll make it louder. volume is kind of a product of pipe length, muffler, and compression. if you have full exhaust to the bumper it'll be quieter than dumping it at the axle after mufflers.

but you're running a relatively low compression stock motor, unless it's straight pipes out the side it's not gonna sound full race car.

headers: TTI or dougs. some people have had success with the cheap summit headers, but you really get what you pay for in this department.