Normally when I do cat backs I do the mock up myself then just take the catback to a muffler shop and have them weld it for $80-$100.
Amazon links to all of the stuff I tried:
U Power 6 inch round muffler https://amzn.to/3f8nvn8
U Power resonator https://amzn.to/2RNksbY
J bend Stainless steel pipe https://amzn.to/3hhN4VD May take 2
Stainless is hard to cut. if you only have a sawzall then use aluminumized pipe https://amzn.to/3uHvvBX May take 2
Summit Racing - Probably the best bet for a twin loop muffler
https://www.summitracing.com/parts/xf...

Or you could say screw it and buy the magnaflow catback muffler https://amzn.to/3fcofaR

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