Jim Bow,
My brother built a flat bed for his 66 GMC...it has been 25 years since he did this....but as I recall, he took 4X6 instead of 4X4 and cut the contour of the frame into the 4X6. That way, he did not need any spacing lumber etc to make the bed mate to the frame. Then, he used bolts down through the 4x6 to the frame and the cross member 4x4's too. Covered it with 1X6s and then made side rails out of 1x6s, three high, long on the bottom, shorter on the top for a streamlined look. The flat bed sat up a little higher off the wheels and I always liked the way it looked.
