JoeR, since you are looking at putting the 66 bed on your 70 frame and cab, its safe to assume you are OK with the obvious differences, the fenders and tail lights. I have never done this swap, but have built a few beds from both year series. Based on that limited experience here is what I believe you will run into with this swap. You will find the 66 bed sits about 3/4" higher than the 70 step bed because of the spacers on the bottom of each 66 cross sill. Several of the bed to frame mounting points will have to be re-drilled in the frame, and the 66 bed will be about 1/2 to 1" farther to the rear than the stock 70 bed, depending on which bed to frame holes you re-drill.
Hopefully others who have done this swap will chime in with more information, especially with regard to mismatch you may find with the step boards, fender braces, and possibly other structure under the bed.
Mark
