If your looking for a really rust free bed, you need to look south and west for the best selection. And expect to pay a pretty penny as well. It has been discussed in the 73-79 forum on how to make a bed using a old bed sides and an 80-96 box. You might still be able to buy doors from Ford, and for cab corners your stuck with after market, making them yourself, or cutting them off a rust free cab (but why waste a cab?).
If you want a longer term post to ask for parts from fellow members, try the classifieds thread at the top of the forum. This thread seems to be pretty general so far. Actual classifieds posted in the forums almost always get locked or lost, so if you 2 want to discuss parts and prices, please keep it to PM's and emails Rules is rules