Like to hear from those who put a 3 in complete exhaust in a Dart (70) .......

I run a full stainless steel 3" system on my Duster (not the same but very similar) but I used a Pypes 3" X-pipe up front, Dynomax UltraFlow mufflers and custom fabricated tailpipes.
Just for the record, the Pypes Race Pro's are the same exact design as the Dynomax Ultra Flow and Magnaflow muffler. Super good design for high horsepower applications that would like to try and keep the sound at least a little bit tame. :p