The packing plates 619395 and 619396 are for use when a greater thickness of body to chassis packing is required than the stated quantity of rubber/canvas pads. The metal plates should be as a supplement to the rubber/canvas pads. The use of additional rubber/canvas pads should be avoided and the more solid packing basis utilised.
It is rare for a body to fit a chassis with identical numbers of packing pads side to side and end to end. So how do you know which parts are right and wrong? Life is very easy if the chassis is bare. All that you need is a flat floor, a piece of string, a ruler and a tape measure. The diagonal dimensions are in the workshop manual (545277). The rest is a simple matter of measurement from the floor to selected points on the chassis.
If the body and chassis are joined, the method used doesn't change too much. You still need a reliably flat floor. Raise the car from the floor and measure up to the selected chassis points. More care is needed because of larger measurement distances. Essentially, if the front end is set up parallel to the floor, the rear should be the same. The front needs to be reasonably correct to ensure steering accuracy. Probably as much as 1/2" of misalignment at the rear can be absorbed by careful packing - this depends on individual acceptance. If there is a problem and it's understood, the body can be built around it. It is suggested that misalignment at the front of more that 1/2" should be corrected professionally especially if other repairs or modifications are to be carried out.
Once the chassis is prepared to the tolerance you accept, it does provide a perfect jig for body preparation. It is obviously a good idea in this case to omit the paint finish until the body is finished. The chassis also provides an excellent transport jig for the bodyshell when it goes for painting, so if it is used for this ensure the clamping bolts move freely in their threads. Pack these threads with greased screws when the chassis goes for painting (or galvanising).