I don't know if anyone makes new shells for something that old. We had an '84 back in the '90's with a fiberglass shell on it. I think the original owner had put it on when the truck was new. I decided to take it off one day (by myself) to clean the back of the cab and it was a LOT heavier than I expected it to be!
IF you can't find new, might watch junkyards and craigslist. On craigslist, you may have to buy the whole truck, but if the truck itself is pretty crapped out, you should be able to get it cheap enough. Pull the shell, a few parts, scrap/sell the rest, you might break even or even come out a little ahead.