well, you will not need to change the x member, the 6cyl mounts can be re drilled to fit the 454, you just have to know where to drill them, otherwise just find BBC frame mounts for a '73-'87 pickup. 4.56 is a 4x4 gear ratio, if the truck is 4x4, you have to change front and rear, and the axles will be Dana axles ( not shure which series ), and not GM 12 bolt.