Pre 80s bed on an 85?

[Mattman2010]

I bought a nice pre 80s long bed (non dually) for $50 and I need to get a rust free dually bed for my 85 (I have a rusted out one with good fenders). I was wondering how you install a pre 80s bed onto a 85. I know that 6 of the 8 holes line up for the bed mount bolts, but I was wondering if you just drill a new hole in the frame, or drill a hole in the bed to get the last 2 bolts to hold the bed on?

Matt