Im rebuilding body on a 77 currently to, and just finished putting on new tail light buckets. That one is going to be some work, Id weld in some angle iron braces to the inside of the bed corners for bracing, and then start trying to pull out that bucket, and the inner corner that is bent as well. The newer replacements are ok but the lower rolled lip that meets the cuarter panel bend/curve does not perfectly line up. If you change buckets, then I would put new lower quarter panels on while your at it to.
I still say strip it first see whats bent and rusted, and try and use original if possible.