Both the dash vents are 2.5 inch. the heater box has 2 different sizes 2 inch OD and a 2.25 OD.
I used a short 3" piece of thin wall aluminum scrap pipe from work 2 inch OD and it slides into the id of the larger 2.25 port on the heater box leaving a 2"OD spot for both hoses. That way I could use the smaller 2 inch hoses up to the vent and used the adapters from
Nostalgic AC - Duct Hose Adapters - Duct Hose & Vents. Had to use very short lengths of the 2.5 hose up high by the vents the challenge is keeping them out the way of wiper mechanisms and such.
you might be up in there a while easier to take the passenger seat out.
good luck