Jane Doh: the 727 wasn't poor in terms of directional stability - just that the latreral stability was very strong. (courtesy, probably, of the amount of wing sweep).
The longer -200 had effectively more rudder 'volume', as the extra fuselage length behind the CG provides additional moment arm for the fin. Surmising (never flown the machine), but this is probably the reason for the 747SP's additional fin/rudder area - the aircarft is shorter, so has less moment arm, though the same wing/engine (basically).