If the pressure plate clutch plate friction face needs re-surfacing, get it machined to clean it up. Make sure the engineering company doing the job records the amount taken off as once done, the recorded amount needs removing from the tops of the 'horns' that stick up to ensure proper clamping force is achieved. If this is not done, clutch slip is inevitable. If using a pressure plate of unknown history, it is IMPERATIVE the clutch drive straps are set at the correct height. Many get this wrong and end up with a slipping clutch for some seemingly unfathomable reason. The following should also be carried out if the flywheel has been machined, or of unknown history also:
Place the pressure plate 'horns' upper-most on something to hold it off of the work surface but not interfere with clutch plate or flywheel
Place the clutch plate in position, with the protruding primary gear spline boss on the underside facing the work surface
Place the flywheel in position
If the drive straps are holding the flywheel up, or are under tension pressing on the pressure plate 'horns', more spacer-washers/shims are needed between the drive straps and flywheel
If there's a gap between the drive straps and pressure plate horns, the spacing between the straps and flywheel needs reducing
If no drive straps are fitted, using a straight edge or drive strap fitted to one of the horns, measure the existing gap between flywheel surface and underside of straight edge/strap, and make suitable spacers to this measurement
Where extra drive straps are fitted, as in racing (3 on each location instead of the original two), it is essential to machine the thickness of the drive strap off of each pressure plate 'horn' and spacer to re-gain correct diaphragm clamping action to avoid clutch slip

The idea is to set the straps so that when they are in their 'as assembled' position, they are not under tension, nor are well clear of the pressure plate horns. They should be JUST touching. Under tension the clutch will slip due to decreased clamping pressure. Too far clear may cause disengagement problems.