what is applied
Bill this unit has an aluminum piston in the reverse input drum. Once you have it apart, look at that piston carefully. You will see a bleed hole in the piston, where the bottom 'waved' steel plate will flatten out against the piston, when this piston is applied.
Close that hole down in size. When new, this hole may not be a consideration, but, after awhile this hole size, because of many reasons, will let too much reverse oil leak thru, creating a reverse delay, or shudder in reverse.
I would suggest a transgo shift kit, a better sun gear shell, maybe the beast sun shell, a kit that provides the ability to install more clutchs in the 3-4 clutch pack.
There seems to be no end [ relative to parts ] to what you can install into the 4L60e trans, as you strive to make it better. Good luck!