I have the same issue with a fresh 1965 727. It takes forever to shift into 3rd when cold. Once up to 30mph or so, I can shift to neutral and back to drive and it goes right into 3rd. The problem is not as bad when the trans warms up. Had the pan off to assure that the shift linkage was perfectly adjusted and the kickdown is a new Bouchillon adjusted properly. I am leaning toward a valve body issue.