727 Kickdown Linkage Adjustment - HELP!

I just p/u 70 duster which has a 340/ 727 tranny. It has a b/m shifter which needs adjustment. It shifts fine from 1 to2 but won't go into drive until 50mph. What do I need to do to make it go into drive sooner. The last time I adjusted a tranny was in 1976. I seem to have forgotten how it's done . Any help is appreciated. Thanks