You use a tape measure!
Aries muffler will be 29 1/4 from front seam to back seam on the tapered muffler. Reproductions will be shorter.
The make a steel muffler and a stainless steel muffler.
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That photo is from the Aries muffler website. Aren't they claiming that they are making mufflers from original blueprints?
Do you have a photo with all the dimensions photo shopped into it?
A simple way to tell is to inspect the weld along the side of the muffler. The original, and Aries mufflers, have a wide, rather slopppy weld. If you were at the MARC meet in San Diego and saw and heard Ron Rudes special delivery running it has an origial muffler on it.
i have a NOS on on my early 28 truck, got it off ebay for $150
John