Start by copying aftermarket tubing diameters, if possible. There's plenty of info in the interwebs about diameters for aftermarket exhaust systems on the newgen. Start there and mimic a little bit.
Obviously don't make it 1/2" diameter for the combined section and you'll be fine. Plenty of people have done all kinds of weird and strange looking exhausts for the pregen. It will run, may just require a little rejetting.
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