That 1965A cab is probably 16 ohms, but it might be 8. If it's 16, wire a pair of 8 ohm speakers in parallel in your 2x10 to match at 16 ohms. If your current cab is 8 ohms, wire a pair of 16 ohm speakers in parallel in the 2x10. With two matching cabs (ohm-wise) you'll need to set your amp at half of that value, either 8 or 4 ohms.
That 1965A cab is probably 16 ohms, but it might be 8. If it's 16, wire a pair of 8 ohm speakers in parallel in your 2x10 to match at 16 ohms. If your current cab is 8 ohms, wire a pair of 16 ohm speakers in parallel in the 2x10. With two matching cabs (ohm-wise) you'll need to set your amp at half of that value, either 8 or 4 ohms.