Hi,
For a '78 you should have a right angle piece on the down slope that is welded to the body on each side. Then you should have two extra pieces, again right angles, that screw approximately 3mm from the body pieces and this creates a channel to hold the bottom of the side screens.
If you have anything left of the old ones there should be a U shaped stainless steel strip that goes down the long edge to take the canopy fittings.On the original perspx that was necessary but the Lexan/Polycarb ones are not as brittle and it is nice to have but can be left out or substituted.
I put my last set on a car a couple of weeks ago, so depends on how much of a rush you are in for a new set.
Terry