Not just a B 727-227, it was N406BN. Southwest got it in an out-of-court settlement with Braniff for Braniff's anti-trust violations against Southwest in its early days.
It was an experiment to see if they could use a larger aircraft. At the time the B737-2H4s all had 118 seats so they put 155 in the 727.
They elected to forego larger aircraft, it was too hard to turn them in 10 minutes and the hassle at getting crews type certified was a pain. In addition, if you had a plane full (155 folks) and the thing broke....you had a heckuva time cramming 155 people in a 118 seat replacement aircraft.