Here are some parts for a 63 fleetside bed from Mar-K . There aren't any bed sides, but there are probably some trucks in junkards with your body style to choose from.
There is a Bed Forum that Mar-K sponsors and monitors. Those guys can answer your questions about fitting the stepside bed on the fleetside frame. (I can't see where it'd be a problem. GM wouldn't have put out two different half ton frames just to accomodate the different beds.)
Once you get the frame straightened out, it should be a piece of cake to assemble the bed yourself.
