We have done several and I'll put a couple of pictures of a 40 chevy pickup in the shop that has a four piece hood like yours. It's not practical to weld the sides to the top because there is no good way to brace them and being long and of thin metal the flexing would soon break the welds. The sides are fastened with bolts or quick release type fasteners. The hood top has room for bracing but if you leave the original center flange it's usually good enough for normal use. If you don't use lead then the weld joining the two top panels must be continuous or the filler will shrink back between the beads; it might take a year but it will. Use a TIG or Mig and weld in 1/2" beads and let cool completely. I allow 8-10 hours to weld together and it will save several times that period by not having warpage. The easiest way to hinge is on one side or the other because the rear of the hood overlaps the top of the cowl so far. Two plain old strap hinges work fine. The are making kits that move the hood up and forward before raising but I think they are pricey. 1940 pictures (first two) at http://s32.photobucket.com/albums/d41/coilover/

Evan