Stock muffler on 64 dart

The stock mufflers and the stock 1¾" pipe were undersized and overly restrictive. Their only benefit was that they were cheap for Chrysler to buy from the supplier. Do yourself a large favor: put in a 2¼" headpipe and use a larger muffler. No need to get a "performance" muffler that will make more noise than you probably want to hear, just get a larger-than-stock, stock-type muffler. See here. Your reward for being smart in this way will be improved performance, driveability, and fuel economy with low noise level.