After swapping out slants in my 79 Aspen over the weekend I noticed my headpipe only measured 1 3/4" OD. I thought the Super 6's like my 79 had a bigger head pipe than their 1 barrel counter parts? The 80 Aspen 1 barrel donor car looked like it was a 2" OD pipe. Would it be beneficial at all (mileage or power wise) to have some new larger exhaust installed at a custom exhaust shop? Or is 1 3/4" more than adequate? If it would be better what size would you guys recommend?

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A 2 - 2 1/4 inch OD exhaust system does offer additional improvement.
If you do a custom system, I would get 2 1/4", just because it avoids any mis-match issues where the pipe meets the manifold.

If you are on a budget and the rest of your exhaust system is in good condition, swapping in a 2 1/4 for the first 6 feet of pipe, right off the manifold helps.
DD