1958 was the ONLY Squarebird with rear coil springs. Ford quickly reverted to leaf springs as the coil setup was steeped in problems. Leafs use a lot less parts, cost less to assemble, are cheaper to produce (since a host of cars across the industry already use the same springs), and there are fewer things to go wrong.
I'll get lots of heat over this but if I had a '58 SB, I would put leaf springs on it. The coil setup is not supported, the parts become sloppy or break and the whole system becomes noisy. No problems with leafs.
For air suspension there is another alternative for leaf springs. AIR BAGS! They conveniently fit between the frame and the center of the leaf spring. This is a good thing because shocks control the ride but springs or air bags control the weight. So... you can get common 'classic car ride' shocks (which are spongy) and at the same time control the suspension with air bags. - Dave