Airbag Kits for 1968/1969 Dodge Dart (actually for Australian Valiant VG Hardtop)

You could buy a 4 link kit and add the shocks and stronger UCA's but those shocks are $900 a pair.
https://www.hotrod.com/articles/home-easy-4-link-rear-upgrade-mopars
Example
RideTech 21140701 RideTech HQ 7000 Series Shockwave Air Shocks | Summit Racing
Example
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Street-Lynx Rear Suspension


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The Reilly Motorsports Street-Lynx system (PN RMS553, $1,995) is a complete kit that converts your leaf spring rear into an adjustable four-link without any major fabrication. Only an upper crossbar needs to be welded in between the existing frame rails.
OR
Call RideTech and ask.