Single key size?

[a50ina50]

My Mustang came with 2 keys. They are both the small square ones and are not original Ford keys.
1) Should the single key set up be the small square design or the bigger oval design?
2) Where can I buy original Ford keys for my single key set up?

[dougphillips48]

The correct key is the rectangular one used for the ignition. You can find original Ford keys on ebay.