Your 351 would be difficult to dual out due to the way the sensors and converters are set up. The best route would be a dual outlet muffler.
Lets start with my recommendations on the headers.
JBA Smog legal headers are the way to go. You can get them so that they will bolt up to a the factory exhaust as well, though I'm sure you will be changing stuff out. They aren't the cheapest mufflers, but you can't go wrong with JBA.
If you want to stay with the stock cats, thats fine. You can get a little more performance with good high flow converters, but the factory system is ok too. the factory cat is not the best, the large size makes them quieter than most, and they can also slow down the exhaust more than a high flow cat will. What replacement you would need would depend upon what your current air tube setup is like.
This part would be fine if you do not have any air tubes, of if there is an O2 sensor in the rear of the converter
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For the dual tailpipes and a great sound, a good dual outlet muffler would be ideal.
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