Tabco used to sell a piece that covered that area but looking on their site I do not see one listed any more..
A place called Auto Body Specialties in Middlefield CT may have what your looking for..they sell repair panels at some swap meets in MA and CT and they seem to have a lot more selection of hard to find pieces..
Your probably better off calling them though--I tried looking up 73-87 GM trucks and their website sucks,I couldn't find any ,but I refused to accept "cookies" too so maybe that's why..most all the rust repair online sellers web sites and photos suck too,hard to tell by pictures if the panels are what you want or not..
1-888-277-1960....toll free..