1975 duster 360 exhaust question

Although that kit will fit a '75, advertising as so will land Flowmaster and Pypes in hot water with the Feds very quickly. Pypes makes systems for the A-Body, in both H and X-pipe configurations. You also have a choice of 4 different mufflers.
As correctly stated above, you will need long tube headers as nobody makes SB downtubes for the A-Body.
Do your homework on the sound you want. FM American Thunder is loud. So is Pypes Violator. Street Pro is moderate and Race Pro is on the quieter side. Their new Turbo Pro muffler is a copy of the Dynomax Super Turbo and is and awesome street muffler.
Pypes