Coil Spring Specialties 632 W. Bertrand, St Marys, KS 66536 in 1994. 913-437-2025. My custom '62 Fairlane Springs (302V8, C4 instead of 221 2spd auto) were $150.
I carry 3 boxes of tools to my country property for working on the tractor-mower-trees-vines and have my 240D maxed out without an extra tank. I've been thinking of putting on one of those caged air bag arrangements like they sell for street rods and lowered pickups, I can't find the brochure but they advertise in street rod magazines. They show up at the street rod nationals in Louisville in August. Stiffer coil springs makes the ride harsh if your tanks are empty. The classic solution is overload (leaf) springs, but the jointed rear axle and unibody design of the 123 negates that as a possibility. If I put air bags on, I would drill the trailing arm which is pretty strong for the lower bag stud, but I would have to weld support plates to the car body floor, because that place is not strong enough to support loads. I might have to weld an angle bar or box beam all the way across to transfer the weight to the "frame" boxes along the edge of the car.