[Jonboy]

The body tub should be the same; most of the difference is in and around the rear seat footwell. The '78 and later K5's used a fold and tumble type of rear seat with a dropped footwell, while the earlier trucks had a fixed seat and flat rear floor from the rear of the front seats to the rear sill.

As far as the frame differences, the front horns on an '81- up frame looks like they are cut off versions of the earlier frame, and there are center bumper brackets. The '79 front bumper would have bolted directly to the frame in the center lower hole. You could cut the earlier frame and use the later bumper. The rear frame horns will be the same, but use the brackets that correspond with your bumper (they are different).