well are you flipping your springs ontop of your axle? assuming your axle sat above your springs to begin with flipping it would give you more axle wrap and blocks will make that even worse. i dont know what your doing this to because i dont know of any GMT400s with the axle ontop of the springs, but if it were me i would just get lift springs for your application and not flip the axle, and if you decide to go and flip it with blocks id higly suggest you do it the right way and make up for some of it with traction bars ( i belive thats what they are called ) but honestly its going to take the same amount of money either way id just do it the right way the first time...