Motor is a 318/ 904 in a 68 dart. 1-5/8 tubes. They all come apart with double slip fit collectors to minimize exhaust leaks. Starter is able to come in/out and oil filter is serviceable. Mild steel but they will be coated
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That looks good!
Your attention to detail clearly shows i like what you did to the first bend on #1 and #2 to get some reasoneble space to reach the bolts.