Nobody, that I have heard of makes body parts or rockers for the 320. Everyone seems to be making their own. If you follow along on Matti's build here on this site, you will get some great ideas, and inspiration. I think he even has some measurements and dimesions you can use to have someone else bend the metal for the rockers if you're not up to it or don't have the equipment. I actually, built my own using clamps, 2X4s and a hammer. Came out pretty darn good too, but I was looking for a show winner either. I just wanted metal in place of the rusted out rocker panels and lower fenders. Sounds pretty much like yours.
Following this link - It's here on this board. By the way, Matti refers to SILLS (it's what we call rocker panels in the US ).
http://www.olddatsunpickuplovers.com/forum23/193.html