thats a good question IMO...i installed the eibach pro-kit on my car when i bought it, dropped it about 1.5 inches. after a few days i got used to the harsher ride and i don't even notice it anymore. i have the stock struts and shocks though. i plan on getting 10.5's in the rear so i'll be looking at tires that size too. (provided i keep the GT and don't get a cobra)
Don't know what the "softest" riding springs would be....just as long as they are coupled with some good shocks and struts to "soften" the ride quality.
If you wanted to maintain a stock ride quality along with lower ride height, you could just heat your current springs. Use a spring compressor to provide force, and then bring them to 400* or so in an oven with slow, even heat. Remove from the oven, let cool, and remove the compressor. The springs will be shorter but have the same rate.
You must use slow and even heat to do this, as torching a spring will just ruin it. Look at it this way- you are currently planning on buying aftermarket springs. If you try this and ruin your stockers you really haven't lost anything...
Apparently FRPP "B" springs will lower the car modestly (enough to lose the 4x4 look), but yet maintain a more or less stock ride softness, even with the stock shocks/struts.
Anyone have pics of Ford B springs? I am very interested in lowering my car, but with the stock 103,000 miles suspension, Im guessing that I will need new everything. C&C plates, bumpsteer, Ball joints, Shocks and struts, Lower control arms, etc.
Make sure to get a progressive-rate spring like Eibach Pro kit. The ride will be noticably firmer than stock, but coupled with some nice shocks/ struts it will ride and handle nicely ... not harsh at all. I originally had Bilsteins in mine ... which are amazing ... but ended up taking them out in favor of a softer shock (Tokico Illuminas set on 2). Other people drive my car from time to time, and no one comments now on the firmness ... they did when I had Bilsteins. The interesting thing is that just yesterday I replaced the stock tires with Dunlop Sport FM901's (same size) and the car rides noticably softer (and quieter) ... it must have a much softer sidewall, I guess. So, now after that, my car handles amazing, but is a very tolerable daily driver. Just my $0.02...