From step 5 of the instructions (I know, no one reads the instructions)
This Backplane kit is applicable to the Phase Linear 400 Series 1 chassis with the 8 fin heat sink structure. Use of the 4 fin chassis is possible but bias transistor Q6 must be relocated slightly (coordinates for the Q6 mounting hole center referenced to the hole center of the bottom most TO-3 mounting hole in the column just to the left of it is [1.000, 0.391]. In English, that is 1.000" to the right and 0.395" upwards from it). This applies for both the left and the right channel. Use of this kit in the PL400 Series 2 is the same as the 8 fin PL400 Series 1. There may be other inconsistencies in the 4 fin early amplifiers that you will have to contend with, Phase Linear changed things up frequently in this generation of amps, things settled down when the 8 fin was introduced.