3 in 400hrs is not good. Are you getting bad vibrations from the motor? Are you running a WOT? Do you balance your blades meaning, are you getting bad vibrations in general.
Ive had one re welded and its been fine since but yeah, its been an issue for many years with Hustler. Had one on an older machine crack as well but replaced under warranty.
Are you replacing the pipe or mending it? I would get it really well welded next time and see how it goes. The heat makes the steel brittle and thin but it should take 100's of hours for that to happen.
I dont know if its possible but Ive thought of a extremely high temp flexible pipe somewhere between the muffler and the engine would fix the problem. I havent looked into it though.
I would also make sure the engine is bolted down properly as well. Ive heard of issues with that too. At the end of the day its vibrations that cause the problem.