My cuzin has been wanting to install a shift kit on his truck (03 5.3 ) what else is needed to make the tranny stonger? and how much will this helpout?

-Bad Black SS

 

Making it shift more firmly, but don't get greedy. A harsh jolt-shifting 4L60-E is a short-lived 4L60-E. Your cousin will find that the instructions give you the choice of 3 different firmness "stages." Use Stage One only!
If he wants the transmission to last as long as possible, do not have the tuner program out torque management.

 

What if he makes his tranny more Heavyduty? like replacing the clutches for some heavyduty ones and stuff like that? how good are every stages?

 

The Stages in the shift kit establish shift firmness. Stage one is considered closest to stock, but still much improved. Stage two is very firm and stage three is for "race" which is kind of a paradox since the 4L60 makes a crappy racing transmission.

There are dozens of high performance aftermarket parts available for the 4L60-E, including clutches. None of this stuff can be done without a full rebuild.

Here's how I would set it up:

With stock second gear piston (aka "servo") Use Stage one as indicated in instructions.
With a Corvette second piston use Stage one per instructions, but omit the one washer in the accumulator housing.

Obviously this makes no sense right now without the instruction in front of you, but once you see it you'll understand.