1st to 2nd @ 20mph, 2nd to 3rd @ 54mph Lokar help needed!

My mechanic/buddy swapped the LD4B intake, 1406 carburetor, and Lokar throttle and "kickdown" cables onto the 318 in my Dart GT.

On the ride home I noticed it shifts rather late (at the speeds stated above). I think I need to get the shift points down to ~15mph and ~30mph. Most of the roads I drive are 35-50mph speed limits, so the 2nd to 3rd shift point really needs some attention.

I believe I need to "lengthen" (have a little slack in) the cable to lower the upshift points. Am I correct in my thinking?

Anyone else have this experience?