225 Dart chugging past 70...

Oh, ok. Then scratch the muffler.
In that case, I would suspect a loss of fuel. Since you have done your due diligence, I would suspect a restriction in fuel delivery, or the pump is sucking air, or, you maxed out the mainjets.
Since delivery is easy to test, I would start there. And if you dump the fuel from the pump into a clear jug through a clear hose, you can check for air entrainment at the same time.
Alternatively, It might be possible to run it up to speed, and when it craps out,chop the throttle and pull over. Pop the top of the carb off and check the fuel level.Choose your test location carefully.