Raptor,
It'll probably be less expensive to do as Grigg suggested and have the sides bent up for you at a local metal fab shop. You may want to use 16ga instead of 18ga sheet metal for the sides for a little more strength.
You may even find that the shop can form the rolled lip for you. If they can't, a piece of pipe or tubing the correct size welded to the top edge is easy enough to do.
If you had to buy 2 of each side and try to weld them together, you'd have to be very careful to get them lined up properly, as well as deal with the possibility of warping.
I was fortunate when I found my 3800 as the bed was in pretty good shape for a near 50 year old truck.
Lane
