Not sure if LMC truck parts sell them or maybe here in the classic parts this is ahard part to find and 73-80 would be all the same for your 1978. There is a few other parts places on the net if these two do not have it to try.
I,ve seen plastic door guard used on some apps some times you can find it at body supply stores check fit before buying it's easy pretty easy to install held on with double sided tape
