Only problem that I see when cutting a 76 and later K5 to make it a full convertible is the fact that the rocker boxes aren't there. 73-75 had boxes under the driver and passenger side to increase the stiffness. Also frames from I think 78 and up and different that the 73-75 so you can't swap them to the 73 if you wanted to save your body.
You can see them on each side of the frame in this picture.
