Starting the dovetail at 6' would give you about a 7-to 8' foot long bed, or a very short dovetail. Where you start the dovetail depends on how wide you want end to be and how much angle you want on the sides. Most dovetails will start at the rear of the tire or close to it, and the slant back to the width of the frame.
With this 6' bed on my '91, the rear is 49" wide, and I started the dovetail about 56" back from the front of the bed.
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