There are a few good shift kits from different vendors. Choice depends on build, HP, intended use and other factors. Cost from $25-$150, and labor at least $100 plus fluid, gasket and filter.
The valve body needs to be removed and mods made to some of the components, and re-installed correctly - especially the little check balls that fall out!
Doubt that you could even measure the E.T. difference with a shift kit.