[dsraven]

if your frame is level then your cab should also be level. with that said it is more important to have the front suspension geometry level that the center section of the frame, if they are not in the exact same plane. the cab can be shimmed as needed to get level but if the front end is out of whack then alignment will be a nightmare.
I have used a piece of straight angle iron or square tube laid under the frame rails and clamped, from side to side, to check the measurement of the cab in relation to the frame. one at the front of the cab and one at the rear of the cab. that way you know if the cab is level with the frame and also if it is hanging below the frame. below the frame equals body work later if a high centered scenario happens to come your way. I don't have a "bagged weed eater", so not sure how you guys roll, but that is something of concern for my '57 build. lots of big speed bumps where I come from. I am going to paint my truck so bodywork later is not really a desirable side effect.