Thank god you don't own a corvette. I swear you will count every brick in the road. Drag springs will be even worse. I honestly don't know of a "softer" spring.
coil overs, and new front a-arms/bushings. I was amazed by how much my ride smoothed out when I swapped my stock a-arms for some FRPP ones with the sealed ball joint. World of a difference
It sure as heck ain't to be found with Eibach Drag springs and Koni struts. Mine rides just as harsh as my prior '87 Formula Fireturd. I've ridden go-karts with softer suspensions. Fortunately, Arizona has some of the nicest, smoothest highway and city roads in the nation...
I'd figure the softest ride you could get would be with some OEM-style struts/shocks of a good brand (Gabriel Ultras seem decent), and a set of 4-cylinder LX springs. You'll lose some height up front, but a lot of drag guys use the 4-banger springs up front for better weight transfer and such, so...
My '90 still has the orig. 4cylinder springs up front since before the 5.o swap....this lowers the front end 1" and smooths out the ride a lot but also takes a lot away from the already crappy handling.
Just a thought ... You could always use springs from a Crown Vic ... but you'd have to install the whole frame and rolling assembly, too. But what the heck ... the box-body models have a 5.0, too, so just swap in your HO motor and there ya' go! A regular sofa on wheels.