HELP!! with my 75 Dart

May not have a hole in it. As said, just a big kink. First ran into plugged exhaust many, many years ago. Car was towed in that would barely run. Just happened to notice the tailpipe was almost completely plugged with dirt. Owner had backed into a parking place with an embankment behind it. Same result if you have a kink pinching the exhaust closed. In a rear-end hit, over the axle is a good bet but I would check the whole system.
If the exhaust is good and it's running rich just since the accident, maybe have someone "competent" check the float level in the carb. Sounds like the "shop" you took it to knows nothing about older cars. Sad but true these days. Several years before I retired, another tech and I were talking about points & condenser. A young tech walked up, listened for a minute, then asked what were points? :p