My 49 has (I'm told by the parts guys based on diameter) older Ford defroster ducts (I forget the diameter, but smaller than 1.75" - took some time to find the right stuff) and a 30's vintage GM Master.
So for mine, I made one by swelling up some exhaust pipe to match the ducting and welding up my own "Y" with larger pipe to slip over the GM heater. Serves the purpose. Wasn't a big job - nobody can see it anyway.
