I wonder if you might want to consider making some sort of flexible mounts between the mufflers and the frame.
I think Harley designed that funky bracket/crossover thing when they rubber mounted the sporty engine, so that the entire exhaust system. including the mufflers, could move with the motor. Nothing on the oem setup is bolted rigidly to the frame.
A short piece of rubber between the bolt on the muffler, and the bolt in the frame so the mufflers can move a bit. Right now your headers are moving with the engine, but the mufflers can't.
This is for a car, but something similar is what I'm thinking about.
https://www.summitracing.com/parts/wlk-36272?seid=srese1&cm_mmc=pla-google-_-...I might be over thinking this. May not be an issue at all. But I think I'd keep a close look at the mounts and the header/muffler joint for a while just to be sure. The rubber mounted engines move around a lot more than than the old sportsters that were solidly bolted to the frame.
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