I have put a 4" suspension lift on two different K5's. The hardest part may or may not be removing the steering arm on the front axle. 2nd hardest part may or may not be the leaf spring bolts.
As said before, soak everything in PB Blast for several days before attempting this. On my 89 K5, we broke a sledgehammer handle while trying to get the steering arm off. On my 86 M1009, we removed the steering arm with a few taps of a ball peen hammer. On my 89 k5, some of the spring bolts were seized to the bushing sleeves. On my 86 M1009, everything came apart easily. The 86 M1009 I started Friday after work. By end of the day Saturday, it was all done. The 89 K5 took several days and sat for awhile because I ended up having to get more parts.
Thoroughly check the condition of the rear leaf springs before you decide on lift blocks, shackle flip, or lifted springs. I did not do this with my 89 k5. While installing the ORD shackle flip, I was removing some rivets and hammering. A large piece of metal fell to the ground, it was half of the main leaf wrap that goes around the spring bushing! Springs had to be replaced anyways. I should have just bought lifted springs.
Good luck, be safe, think things through before you climb underneath the vehicle