I'm still trying to figure out where you are going with this.
Ir you are attempting to exhaust the carbon monoxide gas of the exhaust from the boiler then it should have gone straight up through the roof to allow for the natural air flow over the roof to draw the exhaust out. If you are using it for the air intake then careful calculations need to be done to insure there is no negative pressure restriction of the intake of the combustion air due to the directional changes or flow and piping length these are different for natural gas or propane. Compensating for intake air flow restrictions can be accomplished via a muffin fan installed inline to create a positive intake flow this will insure a more efficient combustion.
I once installed a vent a hood over a grill in the food court at a shopping mall due to the requirement that the duct work had to traverse a horizontal distance before exhausting through the roof it had to be clad with several layers of non flammable heat resistant materials and certified by the fire marshal and before he would do that an HVAC engineer had to certify the the design. Due to there being a horizontal section in the duct work the exhaust stack height above the roof had to be increased proportionate to the horizontal distance.
I love the idea of concealing the intake and or exhaust in the soffit as you are doing as long as every characteristic of the design is taken into account.