Any problems putting 3’ exhaust on 70 Dart ?

All depends on how you route them. Here is a '76 Truck H-Pipe Dual Exhaust. Going over the rear end with the duals from here was no easy task. Factory plastic gas tank on the driver's side, so have to run the dual mufflers down the pass side.

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Duals out the back with resonators at the rear bumper. Sounds nice and mellow.

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Here is a '72 Duster 318, Stock Manifolds, 2 1/2" TTI X-Pipe Dual Exhaust that ended up with DynoMax Mufflers to kill the interior drone.

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Getting around the Gas Tank and Rear Shocks is going to be the trick, then you are going to have to find out how you are going to hang them out the back.

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