Most people chunk suspension all together for drag units. It's easier to change out the entire shock when you want to go back. If you use it only for drag racing, you want want the "shackle" type of lowering kit because that can allow the bottom of the shock to flop a little bit.
Most people remove the shocks and lower the vehicle down to where the suspension is totally flat. Using a jack to support the center skid plate you can adjust the height to get it as low as possible without the frame actually touching the ground. Then measure the distance between the shock mounting eyes and cut a piece of 1" square tubing to length to fit.
Once the front is sitting down with no suspension, you can lower the rear doing almost the same thing. There is one more consideration with the rear end though, you need to make sure that the chain isn't put in a bind by lowering it too much.