I looked into this, the short answer is it is impossible. Or possible but expensive piecing together.
Your either better off finding a cab and chassis DRW, and modifying your bed inner fenders, or getting a set of outer fenders and the truck version DRW.
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1985 GMC K1500 long bed, 6.5 TD W/ NV4500, NP208
1991 GMC K2500 6.2L NA W/ 4L80E