after working on the original 71-73 Heating ac unit myself it would be difficult to include a cabin filter. you could have a filter easily when they unit is in Cabin air recycle mode.
putting the cabin filter in the cowl i would be worried about water. the cowl drop into the original air unit is a straight drop down there is plenty of room for the cabin filter but there is no baffling system i would be concerned about road debris and water clogging up the filter and rotting out the cowl area. putting a mushroom cap over the cowl intake might be a good solution. this way the air must travel up the sides of the cowl intake then you can put a filter in the throat but you will have to design the cap so it will not interfere with the windshield wiper motor.
alternatively you could place the filter in the center air vent assembly the air coming into the car would not be filtered but the final air entering the cabin would be filtered.
something to think about is maintenance. taking the air assembly in and out is pure hell you have to just about remove the dashboard from from the car body as one unit just to properly service the fan housing and air climate control system. if you buried the filter in between the fan housing and the heater core and evaporator core housing your looking at a full day of removing parts just to get to the housing without breaking anything to change one filter.
if you locate the filter in the center air deflector housing then you will at least need to remove the center dash panel, and radio to service that. it would take some hacking but you could make a fiberglass modification to that center vent piping and allow for a slide in cabin type filter that at most would take 2 hours to change.
designing a mushroom cap made of fiber glass and charcoal filter insert might be the best approach, you would just need to remove the hood, and pop off the plastic grill to get your hand inside the cowl fully, then you could measure the cowl and start fabricating something that would just be press fit over the cowl hole without disturbing anything inside the car.
you could adapt this cap to be used on a Heater only car as well that uses 2 fresh air intakes one on the passenger and one on the driver side. care may be needed for extra clearance on the drivers side.
one the prototype is done you should be able to install this cap over the cowl with cabin filter without taking the hood on and off, you should be able to access the cowl just through the plastic leaf screens.
you may have a market for this item, i can tell you when i rebuilt my 40 year old A/C heater box the bottom was full of road debris and acorns.
well just thinking out loud
