Cheap REPRO mufflers are not the same as Ford mufflers.
If you take time to read Aires muffler info you will see the difference.
http://www.amuffler.com/
From website, this only part of the info available:
1. The mounting flange to the exhaust manifold requires two operations, as the size of the flange is such that the material must be annealed after the first pass to avoid rupturing the metal.
Other mufflers on the market only make one pass and the resulting flange is not large enough to form an effective seal. It also vibrates loose and leaks profusely. There are many fixes to this problem, but the real solution is to make the flange to specifications.
2. Our tubing is "mandrill bent." When tubing is bent, it must be collapsed slightly on the inside of the bend, in order to be forced around the bend.
This greatly reduces the cross-sectional area and restricts the gas flow. Ford mandrill-bent all its muffler tail and inlet pipes; we do the same. It requires taking the conventionally bent tubing sections and placing them into a die of the correct diameter and radius. While restricted in the die on all sides, round steel balls are forced through the inside of the tubing, bringing it back to a full round shape by brute force. This is a very slow process and greatly adds to the cost of making these pipes. Today this process is only used for precision tubing applications such as aircraft and medical requirements.