Not sure which flat piece your talking about.

The flat part that that has the pipe sticking out of it.

My experience with duals. My last MM was a long tail GD with a 27 hp Kohler. I run 100% of the time in salt water and that first muffler didn't last long. I'm a welder and fabricator for 40+ years and made my own set of duals using 316 16 ga. and stuck 3" Supertrapp cans on the header pipes. My single support tube did crack and after changing out the support tube to one larger no more cracks. What did happen was the engine vibration loosened the end nut on the baffle end cap and lost those guts to 30' of water, safety wire was the answer to that one. The duals looked great on the long tail but on my 36 PD this winter I will build something different which I've been kicking around.

Well, here is an update. Got the boat home today and looked closer at the pipes. On the side with the crack, the baffle in the end of the pipe is loose and just jiggling around. On the other pipe, there is no crack and the baffle in the end is tight, the welds are still holding. But, I can turn the whole pipe! Where the smaller diameter pipe slips into the larger muffler portion is loose. The other side does not do this. I'm guessing, and I'm definitely not a mechanic, that this pipe not being firmly attached may be what's causing that side to run so hot? Possibly more air getting into that muffler causing the flame that I saw the other night? Could that be the case or am I totally off track?