The true reason for the four link change was because of the way the rear axle flexed and the stock upper control arm connections that were ripping the car apart! Even the additional straps added to the upper arms were to keep the rose joints for tearing apart. I have a full set of TR8 FIA Homologation Papers and no mention of dimensions just part numbers and references for the upper and modified lower arms.
Michael
1980 TR8 FHC #0020 Rally Conversion