While I am certainly intrigued by the idea, it's rarely cost effective to convert a 2wd to 4wd in any way. Just save your pennies and buy a 4x4.
As for ride on the street, my constant wheeling buddy's rig is a 79 ford with the radius arms and it doesn't ride any better on the street than my nice flexy leaves. Now if you're running clapped-out negative arch stock springs, yeah the coils will be an improvement, but so would a new set of leaves for a lot less money.
He (the Ford guy) also destroys a set of radius arm bushings about once a year (even good aftermarket ones), so be prepared for that too.
As for a link setup (if you're intent on coils, it's the way to go!) if it's built correctly, it'll be GREAT on both street and trail. But not cheap.