Actually you drill the crossmember for the bag plates
You need the following for a simple front bag setup
Bags, upper bag plates, 90 or 45 degree air line fittings, grommets, bolts & most people use spacer cups under the bags to keep them from rubbing on the LCAs
Drop spindles are a good idea as well as shorter shocks
For the rear I would run the bags in front of the axle, the further back the bags are from the pivot point the less travel you are going to have, on the flip side it makes the weight handling of the bags closer to what they are rated at (not needed for a 1/2T pickup) & it can make it ride rougher
If you mount them in the stock coil spring location (some people move them forward a lil bit) then it will ride better
Bags in place of stock coils
Also, if you have the skills to keep everything level & aligned correctly, then I would Z the rear instead of doing a step notch
You dont run into the trailing arm to frame clearance issues with a Z
