Had my 96 sierra ls swapped upgrades truck Norris nsr cam ls1 intake and fuel rails with 42 lb injectors. Running fine for about 5 miles then begins to sputter and run rough. Shop that did swap which was a zero miles swap engine rebuilt retuned engine but still running rough after about 5 miles . Mechanic states that exhaust is causing problem . Took off exhaust from cat back . Still having same issue. Is mechanic correct that exhaust is the issue? No muffler from cat back just a straight pipe running about 2 feet.