Replacing Humidfier On A Thermopride Furnace

Hi All,
I'd like to replace the drum style bypass humidifier on my Thermopride with a steam unit (Honeywell He440). Besides the fact that the water feed needs a good water filter (comes with an RO filter), I like the idea of being able to add humidity when the furnace is not pushing hot air. My questions are:
1. The current drum unit is installed in the plenum. The Honeywell specifications call for a horizontal under duct installation or horizontally under a duct extension. Could an extension be built next to the plenum to allow for both a horizontal installation and the use of opening where the existing bypass unit is located? The only issue with just patching the existing opening and installing the unit under the supply duct is that there are two supply ducts coming off the plenum each of which supplies hot air to half the house. Many of our doors are kept closed to prevent cat hijinks.
2. The furnace also functions as a heat exchanger/cold air mover for my air conditioning. Does this cause any compatibility issues for the installation.
Thanks for any information you can provide.

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