Even after all the work of installing the dual exhaust, I really doubt the engine or you will know its on the truck. It's a lot money just for noise, and if you haven't been around a straight six with glass pack mufflers, I suggest you ride in one before investing the money.
Heat up the manifold around the shaft on both sides, let it cool, then do it again. Add candle wax to the shaft the second time, it should melt and flow into the gaps pretty fast. Add the wax once you shut off the torch.
