Your thoughts are contradictory. It's true the Mark VIII is a money pit, absolutely, no argument there. But if you're going to throw out the thing that makes it a Mark VIII that's the point where I ask you if you're too deep to bail, what's the difference? Put your Mark VIII on springs and it's just a Ford Thunderbird with a slightly up-tuned version of the 4.6 Modular. Get a Thunderbird or a Cougar, less crap on it breaks compared to the Mark VIII's annoying little issues that pile up.
To gut the entire air system and replace it with even a poor to marginal quality spring system is going to be at least $450 if you do every bit of work yourself. Same amount of money or less can buy you all new bags at all four corners, albeit they won't be Motorcraft brand.