later and harder switch with shiftkit?

Hello, do you have any experience with shift kits? My engine 383 has about 400 ft lbs and I ordered a Turbo 350c from the USA. The gearbox switches softly and at KD it takes a slurry of almost an infinite second. The transmission switches from 1-2 at 5000 rpm and the 2-3 already at 3500 rpm. I would like to continue to switch in D automatically and only at KD hard and at 5800 rpm. Do you know possibilities for this or are shiftkits (which) suitable for this?