A few thoughts.
A 99 engine will be tuned for non ethanol gasoline. I am not sure if this is a MPI or a carburetted engine, but either way, it will be running lean. Lean and hot probably contributes to this as there is excess oxygen at the siamesed ports. More so than a properly tuned system.
If you can re-jet or change program or injectors, that isn't a bad idea for this port and the exhaust valves. A little bit rich wouldn't hurt. If it has carbs and you can just go up a bit on the main jets, that might be the ticket. Without excess oxygen in the exhaust, that erosion and oxidation will slow down considerably.
And I am with Bushwacker on the muffler putty.
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