exhaust size ?

[lukky 71]

im building a 71 camaro with a 468 bb. flowmaster offers 2 kits. one with a crossflow 80 series muffler that tucks over the rearend. the other has 2 flowmaster 50 series mufflers that are under floorboards. both have mandrel bent 2.5 pipes. which kit is better and is mandrel bent pipes better than 3" bent pipes. this engine is 550 hp. i know it needs to flow well. thanks for any help.