Yu can just replace the 10-50r with a 6-50r and cap off the neutral with a wire nut.
Not necessarily. If the box is ungrounded that would not be safe.
If it's fed off the main service panel, you could just strip the white insulation for it's entire exposed length, leave it terminated to the neutral/ground busbar and use it for the ground.
If if it fed from a subpanel you could do the same thing, but you'd need to terminate it to the ground bar.
In either case be sure to bond the ground to the box if metal