They don't offer what would commonly be called a "shift kit" but they do have "shift improvers". B&M offers one for the 4T60E and you can get it at www.summitracing.com
. A little info follows:
B&M'* ShiftPlus shift improvers use vacuum or electronics to improve the characteristics of your transmission, and let you change settings as you drive. Great for performance, towing, and RV use, these shift improvers simply recalibrate your transmission in one of two ways. Vacuum-style transmissions, such as a GM 4T60E, simply recalibrate the hydraulic system through the modulator circuit to provide firmer, more positive shifts. Electronically controlled transmissions, such as a Ford AODE, simply recalibrate the transmission'* hydraulic system electronically to give you a firmer, quicker shift for better performance and longer transmission life. A flip of the switch allows the transmission to operate in the original factory mode at level 1 (firmer than factory) or level 2 (much firmer than factory). Either unit installs in minutes with a few simple hand tools.
Its part number BMM-70383 and its about $43 bucks. They don't offer a valve body shift kit because there are service b'* suggesting replacement with an OEM upgrade which will be your "shift kit" in the classical sense. Any respectable transmission shop should be able to do this upgrade. Either way you go it should shift firmer.