[Frank Miller]

Is it slipping in all gears or just 5th. Maybe a borderline pressure plate that is not doing well with the extra load. You did properly tighten the pressure plate bolts, right? If you take it apart I would start with snugging up the pressure plate until it just holds the pressur plate and then measure distance from the plate to flywheel to get an idea how much it is compressing. If you have an extra clutch and flywheel maybe you can measure the torque required to tighten the pressure plate to compare spring tension. If there is a truck clutch and brake place near you maybe they can help you determine spring pressure.