There's a set of Koni lowering springs for tbird/cougars on ebay. I had the same springs on my car at one time and they offered a slightly stiffer ride and a small amount of lowering (1.5" I think). I would have kept them if they would have lowered my car a bit more. Overall they were a great stock replacement without the harshness of springs that offer a lower ride height. I don't know the seller, I was just searching for koni stuff.
http://cgi.ebay.com/ebaymotors/Koni-Sport-Springs-F-R-Thunderbird-Cougar_W0QQcmdZViewItemQQcategoryZ33590QQitemZ130000174748QQrdZ1QQsspagenameZWD1V (http://"http://cgi.ebay.com/ebaymotors/Koni-Sport-Springs-F-R-Thunderbird-Cougar_W0QQcmdZViewItemQQcategoryZ33590QQitemZ130000174748QQrdZ1QQsspagenameZWD1V")
Good price... That's half of what I paid for a set of Suspension Techniques springs
$56 shipping to me, ouch!