Be sure that the rouing path for the "Dual" that you will be adding is clear of RV-type additions like tanks, genset bay, etc.
When our 83 Ford E350 needed exhaust, I investigeted Headers and a High Flow exhaust. I decided to repair the exhaust manifold leaks and go with a 3" single "cat-back" system from Flowmastermufflers.com - In investigating Headers, I found they should be "Long Tube" for the low RPM high Load application we have in RV use. We need bottom end torque not high end revs. Mose headers are short tube, so I'd say satisfy yourself that the headers in this kit are LONG tube. FlowMaster pointed out that a Dual system should have cross tube between the branches for best performance. It can be added, but the Summit kit doesn't have it. A 3" single exhaust will flow at least as much volume as 2-1/4" duals.
I waned to upgrade exhaust on a 71 Dodge van we had. At the time, Sears offered a kit like this Summit kit. Excapt maybe it came with "turbo" mufflers. I installed it. Headers, Pipes, Mufflers, Tailpipes and we took a trip. It didn't seem to have any more power. Motor box was so hot I had to wear boots to drive it. Mileage dropped. Noise was excessive. I was glad the wife wanted me to try a Sears kit. Took it all off and returned it. Ended up with stock manifolds and crossover stepped up into 2-1/2" with a school bus muffler.
Your project is do-able in concept. Many ARE happy with Headers and Many swear by Duals. Just research the details.
