Go with a Dodge Dakota IFS front end. Then you can throw coil overs, air bags or whatever the hell you want on them. IMO the old straight axles are fine, if you have the Twin I Beam like my 66 F-100 has then they are fine for a work truck but for street handling and a low stance they are junk. Twin I Beams are heavy duty though and that is why I mentioned about a work truck. They wear through tires. If you want your F100 to handle like a car then the Dodge Dakota is the only way to go. Do a search on the Dodge Dakota front end for F100 and you will find a ton on them. Industrial Chassis Inc I believe is the name of the company making the kits for these trucks.
Check out this thread on another forum.It is interesting: http://www.ford-trucks.com/forums/866959-61-unibody-progress-pics-crown-vic-ifs.html