wamabam s

hello everyone, had some questions I have a 2016 chevy silverado 4x4 with a 6.5 inch zone lift and was wondering the blocks that came with it are tapered and I belive that is where my vibration is coming from. I bought some new flat blocks to try but I'm worried that my drive shaft won't be long enough. I guess my question is 1st how much is acceptable for my drive shaft to be slipped out and 2nd should I even run with flat blocks ?