I've noticed when installing new exhaust pipes of any kind, alignment can get it touching the floorpan. Loosen the flanges, pull the pipes around. You have to position the system so it has room to grow when it gets hot. It'll get 1/2" longer so it's not good enough to just let it hang & check when it's cold. You have to think about what it's gonna hit when it grows.

It's also possible that your engine mounts are getting soft? When you're decelerating the engine wants to 'lean' forward. That lifts the exhaust pipe up & closer to the floorpan.