A lot of vintage 15x10 slots only have 3.25-3.5" of backspace and a lift is required to avoid wheel opening contact. Your use of air shocks seems to suggest to me this may be the case in your application. This, combined with a bent leaf, and the rock hard ride, suggests you have other items to consider before simply changing out your shocks.
First would be a wheel change to get the tire all inside the well opening. You didn't mention what size tire you have, but I assume its a minimum of a 295. To do this with a 10" wide wheel will require 5-6" of backspace, which will require moving the springs inboard. Inboard kit will run a couple hundred bucks and labor to install, weld, bolt up.
To avoid moving the springs in would require a new rear wheel. If 15", then a 15x8 with 4.25-4.5" of backspace is the ticket. If you want to stick with the classic dish style, American Racing does make a 17" diameter version. Get one in 8" wide with a 275/50r17 and you can have an old school look with a new design tire. Whether or not you change all 4 is up to you, the look you want, and how you use it. This change can run from a few hundred dollars for some used wheels to a few thousand depending on wheel and tire selection.
With a bent leaf, you will want to replace the springs. You could try putting a single leaf pack on it, but then you will most likely have mismatched springs side to side, which is a hazard unto its self. So you're looking at new springs sets. $200-300 here.
Then you can address ride quality with shocks. Bilstein or Fox are the best non-adjustable units out there and will run $100-180 per shock. Great if you prefer a set it and forget it arrangement. If you don't want to pay for high end shocks and you are retaining your stock t-bars and leaf springs, you can get away with gas charged Monroe Magnums or similar. I'd suggest avoid KYBs though due to their high pressure nitrogen charge. If you want to tinker with shock settings, then you want to look at some adjustable units. These will run $100-1000 each depending on the brand, materials, single, double, or triple adjustable modes and other features.
As is often the case, one thing leads to another on these cars. This is a classic example.
