I'm pleased with the two different sets of Tuff Country springs I have used on my 69 K20 and I would recommend them. However, I have not used any other brands so I can't knock the other brands. I agree with Hoods69BadBowTie that good shocks should be considered. I wasn't very impressed with the Tuff Country shocks that I initially purchased. I now run Rancho RS9000's and like them alot.
When I initially lifted my 69 4" it required the addition of a cv-joint for the front driveshaft. You will need to get a dropped pitman arm or otherwise address your steering when you do a 4" or 6" lift. Rear driveshaft probably will be okay, but you won't know until you do the lift.
I like the service offered by
www.offroaddesign.com they know Chevy's pretty well and are willing to help answer questions. The offer greasable bushings which I used on my 69 K20 and the front axle of my 70 K5.
You will also need to keep an eye on your front rubber brake lines. You will need to lengthen them in some fashion.
Hope the above info helps ya.