[Richard]

Having been there done that on the Y junction. At least on mine, the Y was a custom fabricated piece made from 2 90 degree exhaust elbows, I had to slice one of them with the grinder, and weld it onto the other to duplicate the the orig.

All that to support your alternate plan. That thing was a pain, plus you have to be creative on how to get the existing pipe that exits the roof removed.