Are you looking at the Haynes 350/450SL manual? What page does it say this? I'm not refuting you at all -- I'm just curious now!
BTW: I don't notice any sort of lockup "feeling" with my '79, it feels like a garden-variety 3-speed auto. However, 3000-3200 seems on the high side at 60-65. Perhaps the earlier SL's had a higher rear end ratio...I dunno.