Find the cross bar that lines up with holes on the angle brace on the bed side near the rear wheel well. You might have to use a clamp to pull the bed sides together to get the holes to line up between the two parts. Once aligned, mark the bolt holes. At this time also mark all the hole locations for the bolts that go in the angle braces on the sides of the bed. Also mark the hole locations on the middle boards by the front of the bed box. MAKE SURE the boards all space out properly to the holes in the cross braces!
measure the hole locations off the frame CAREFULLY! Check squareness of frame as well. Take your boards and measure out the hole spacing for the rest of the bed mounting bolts. These bolts should all line up on a line at the same distance from the edge of the board. You SHOULD have one of the measurments line up with the second cross brace holes you marked towards the rear of the bed near the wheel wells. You can take your old bed boards and lay them over the marked hole locations as a double check. The bed wood WILL change in width from cracking, warping and shrinking, but over the length they should stay close to what you need. Carefully mark all your holes with a center punch and drill out all the holes. Then using a router and a straight edge route the grooves for the bed strips, mine were 5/8" wide and a little over 1/8" deep. I made a jig to use a router bit with a follower bearing to route out the holes for the large 3" washers for the bed mounting bolts.
next step: paint (for me) or stain and lacquer