Not a bad idea & I've considered the same (keep the OE stuff for the next person in line if they want it).
I was looking @ the aftermarket heat-only case as a less bulky bolt-in alternative to adding an updated HVAC system (Vintage Air/Cold-Air Products/or similar) that doesn't rely on the vacuum actuation/control pods & actually moves some air. I would use the blower mounting spot as the hose bulkhead mount for the under-dash set-up. Smaller than the factory evap case; not just a flat plate that looks out of place on these cars, & not re-inventing the wheel.
