128 3p Rally Car

Hi,
Does anyone know how to lower the rear of a 128? I did one twenty odd years ago, I remember it was quite an easy thing to do , however I can't figure it out now!
My plan is to convert the car to group 2 rally spec and (gulp) maybe do some post-historic rallying. Finding specifications ( eg for engine mods) is very difficult, would anyone have any info that could help? I've found a set of gp2 arches and rear spoiler, also a few places who sell close ratio gear kits and lsds, once again I don't know the correct ratios/specs the group 2 cars used to run........
cheers