Do you plan on having a cross over pipe installed as well? If not I would just keep you factory y pipe and either put in a highflow cat or none at all and do a single muffler.
Adding true duals on the 4.3 with no crossover pipe makes it sounds very strange and is a waste of money to get done compared to doing just a single muffler. You can do a single muffler in and dual out retaining the tailpipes you have running into your rollpan now.
On my 4.3 blazer I plan on doing headers and alittle larger than stock piping to one muffler.
ON my v8 s15 I am going to add an hpipe in it to help out.
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