What is this metal bar in my 1950s electrical box?
Those are the nails that fasten the box to the framing. I'd slide them free with a locking pliers, remove the box, and install modern "remodeler" boxes with flyout mounting tabs. If you trim your plaster/drywall very carefully you'll never know it was done. 

You may also be able to find more compact GFCI outlets that are designed for such boxes.