1967 Dart 273 idling erratic

The exhaust is not causing that. It's more than likely something associated with the carburetor linkage, or possibly transmission kickdown linkage, if it is an automatic. That could have been disturbed possibly when the exhaust was put on. I would also check the primary and secondary linkage and butterflies on the carburetor to make sure they are closing all the way. Make sure the throttle cable is also free and the throttle return spring is present and accounted for.