How effective are the under dash AC duct outlets on a original car?

If you really need some outboard vents, you might be able to tuck some smaller diameter hoses along the underside of the dash and use some underdash vents like these from Classic Auto Air at the dash ends. Just tap the hoses into the factory center vent. That way you could avoid trying to fit hoses around the instrument cluster, radio, HVAC controls, wiper linkage, and glove box.
A/C Vents | Classic Auto Air - Air Conditioning & Heating for 70’s & Older Cars & Trucks.