The stock adjusters have enough travel to bottom out the upper control arms on the hard stops on a gmt400.
Any kit advertising "keys" is best avoided as its more about grabbing your cash than giving you something that makes a difference.
They advertise them like this because the newer style trucks can use reindexed adjusters before bottoming out. The uninitiated don't usually know they aren't needed on a gmt400, but it sure makes the kit sound like all the rest on the market....
Pair of lift $20 rear lift blocks, nearly maxing out the upper control arms and an alignment will get the same results.
You will need an alignment because changing the ride height will change the camber. It will also effect the toe in.
It will ride rougher (or firmer if you prefer) due to the reduced leverage angle the lower arms will have on the bars.
Oh,one more thing you should know; you'll mess your shorts first time those upper control arms whack the stops on a decent sized bump at any appreciable speed....
Chuck on another 20 bucks to the total for new undies!