Meh.
Run 3 inch.
It will be fine.
You won't loose enough to care about.
You are not designing it to pull the n-th amount of power.
If you can, buy a universal dual exhaust kit from Ebay.
You can do it yourself and have the satisfaction afterwards of knowing you did it.
We do it to all the cars in the driveway.
If not having a welder is the issue, you will save that much by doing it yourself.
So buy one.
We did and have built several exhausts and other things with it.
This is the front half of the 3 inch my son put on his 1949 truck with its little 307.
It has plenty of power for what it is.
He will soon be putting a mildly built 350 roller cam engine in the truck.
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Now the truck has a full 3 inch exhaust that runs out under the bumper.
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For my brothers 1972 truck we did the same DNA exhaust kit from Amazon and assembled it in the driveway on a late Sunday night.
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The end result driving out of the driveway for the first time.
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The drive by...
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We also did a 3 inch single under my daughters 1992 Camaro as well as a tailpipe and muffler on a 1976 Nova.
My 1979 Silverado will also be getting a 3 inch exhaust to go behind the 454 that is in the engine bay.
The mandrel bend kits are done quite well and should last for many years.
You can get them in mild steel or stainless.