It should kick in before that, atleast it did in my 71 Dodge charger.
I was thinking 45 - 50. Is the Vacuum modulater good on the side of the trany.
It's got the vacuum line that runs in to it, you may want to replace it if it's been sitting a long time. They are pretty cheap make sure the vacuum line ain't broke or cracked or even pulled off that can mess up the shifting too.
Kurt