A general rule of thumb is to start with a primary that is 10% larger than your exhaust valve and work back from there. If you run full length headers, a reducer after the collector can often help average torque and hp by a couple of points. I'd consider a step down at the collector to 2.25" going to a cross over and then mufflers with a 2 1/4 inlet and a 2" outlet and follow that to the talipipe.
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