Are you planning a Willys original 2 wheel drive front end or an after market Mustang II style? The aftermarket stuff would give you disc brakes.
But anything is possible, given enough time or money. Shouldn't be a real difficult change, much easier than going from 2wd to 4x4.
4X2 in to a 4X4 frame?? That could be rough. I don't know. If you are talking Front End Clip (as in body parts), yes it's possible. The only problem is that you have to cut out the inner fender wells to clear the shocks. Other than that, all body parts are the same.