I am sure you could make it fit, but the hangers would be placed on the exhaust system to work with a Dodge. They may or may not line up or even be close to your GMC.
You could go with the MBRP S6004AL - 4" Aluminized or the S6004409 - 4" 409 Stainless systems that are labled as Offroad systems. They come with everything from the downpipe back, but offer no place for the cat. But you could make up something there to fit your cat into. Might be easier to go that route rather than the Dodge way.
Two things come to mind....
A) The cat internals, even on the 4", may be more restrictive than the 3" front pipe.
B) Enough backpressure reduction to make it worth the cost and effort may still throw EGR codes.
IIRC, a couple of guys towing heavy with either straight-pipe catbacks or MBRP's have thrown codes, meaning that the system is going to be pretty sensitive to reductions in backpressure.
The cat only causes you approx 15% performance/EGT difference as compared to a 4 inch cat less system. Get the MBRP LBZ system and the front pipe and then bolt in back in when you need to. The Dodge cat is larger in diameter, but the density of the comb is slightly tighter so my guess is the overall is about the same flow wise.
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