As a rule, you wouldn't replace either of the two mufflers with genuine replacement units. At some stage, I priced the cost of both 6.3 and 6.9 mufflers and if I remember correctly, the cost was horrendous. Say about two grand plus for just the rear muffler. Anyway, please don't quote as my info is several years old.
It's my understanding that the metal used is mild alloy steel in the interests of longevity. So, what is their working life ? I have just examined a rear muffler, part no. 116...