The "leveling kit" is nothing but torsion keys that are indexed differently. Tosion keys will do absolutely nothing for gmt400 trucks as am92 said, so to answer your question clearly,
No. The "leveling kit" is not recommended to be put on with a lift kit. Actually, I, and any other forum member that knows, dont recommend the "leveling kit" for a stock truck since your stock keys will go as high as the suspension will allow it to regardless.
I have absolutely no experience with BDS or Cepek lifts, but I have heard that Cepek is discontinued and can only be found on eBay or suppliers that have some in stock (rare to find)
I've heard great things about BDS but again I have no personal experience with them.
The RCX (Rough Country) 6" lift is a 5.5" differential drop, meaning that the lift truely only lifts you 5.5" and you have to crank the torsion bars "not max out" to get the full 6 inches of lift.
For cost Effectiveness, I recommend the RCX lift more than any because it's by far the cheapest new not used 6 inch lift kit you can buy, and it is fairly easy to put on.
For lift height, I'd go with Cepek if possible since it is a true 6.5" lift meaning the differential is dropped 6.5". I do not know the differential drop measurement of a BDS 6" kit but I do believe it is a true 6" kit meaning the differential is dropped 6".
I see you're 27, so I'm guessing you'd rather go with a cost effective method and for that, I say Rough Country aka RCX.
Also, as always, I recommend you either check or have someone check the entire front suspension for wear and tear and replace any worn pieces before or during the lift install. The reason is, when you put the lift on, it causes more stress on those parts, and if they're worn, you could end up in a bad situation like having a wheel bearing separate going down the highway at 75 mph.
And also, depending on the tire size you decide to go with, look into regearing the front and rear differentials, if need be, I can do some explanation on the gears and what their ratios mean.
if you go with a RCX 6" kit, I reccomend a 35 inch tire. I run a 37 inch tire but mine are worn down to about half tread and they still rub a good bit off roading and if i hit a bump while turning. with the BDS and Cepek lifts, there's enough lift to clear 37's with no problem but if you go to 37's i DEFINITELY recommend regearing. im running a 4.56 gear ratio on 37's and it does pretty well.
If you need any more help on the subject, I'll be glad to help in any way i can.
And, I first thought you were asking me specifically and began wondering, then i reallized that that's your name too