Remove the transmission. You should also unbolt and drop the flywheel or the driveplate to the crankshaft flange, but do not discard the bolts that come with it; there might be spacers there as well. Screw the flywheel or driveplate,and then tighten the torque of the flywheel or driveplate bolts and all the bolts of spacer if exist. In case of 6.0L engines tighten the bolts in a sequence while in case of 6.4L, tighten the bolts in criss cross fashion and in several stages and to the mentioned torque. Finally, install the transmission.