by switching from a leaf spring setup to a coil spring setup, you will need to change the whole suspension setup. A true coil over setup would be a 4 link or ladder bar setup, but you will need to weld in new frame rail extensions for this setup. To do a nice coilover setup, you will be spending $2k+
I was playing around with my 46 pickup project and I was thinking and easy setup would be to pivot a 2 1/2" thickwall square tube using the front spring bolt, make a saddle on the bottom of the axle, U-bolt the axle to the Square tubing and extend it behind the axle to pickup a Coil-over.
Sounded like a pretty easy solution if you wanted coils.
EDIT: the mustang already has the saddle on the bottom of the axle so you wouldn't need to fab that, my 46 was on the top.