I've never seen an add on to an E-body, we had a '74 Dodge P/U that was aftermarket & it didn't do to bad..nothing as good as the factory, cable actuated, didn't have the recirculating MAX-AIR feature & looked pretty hokey, bulky under the dash..leaked condensate water from day one.
In our area these were typically done by local business & not the dealer...no upgrades were made to the vehicle beyond what it took to be A/C. We had an '89 Astro Van with add on air that was pathetic.