That kit on A/M is listed for GT's not V6's.
The V6 has 2.25" pipes, the GT/GT500 has 2.75".
And the cut and weld sure shouldn't rust out, the oem pipe material is 409 SS, so if you cut the ends off the oem V6 muffler pipes and weld them onto the GT500 mufflers, the only possible rust would be in the welds if the muffler shop uses mild steel filler wire in their welder, and you could coat the welds with cold galvanizing compound or some other rust preventative.
Up to you, only other thing to do is buy something specifically made for the V6 car.
it is listed for the V6 that's where I had found it. I measured the pipe and it is 2.25", did think about getting the GT cat back pipes, but im guessing they are solid 2.75" at that point as well. other thought was doing that and getting a step up pipe to that size and some used GT pipes. didn't think of cutting the muffler pipes, but I think id like to have that for if I ever think of selling the mustang, though I don't think I will.
mines a 14' and I like the look but im not sure if they would work. also wanted black tips.