Nigel,
This is something that I'm in the process of doing, however I had no belts in the back. I don't think that any of the kit you have can be used, what you need to do is phone around the breakers (PHSportscars, Douglas Valley, PorschaPart, Prestige Salvage, Lodge Sport etc) and buy a set of 3 point rear belts and shelf if you can get it.
My car is an 86 MY, if you remove the rear shelf you should find the upper mounting point under the sound deadening already drilled and tapped. (there was a perforated section in the soundproofing in my car, I just cut this out). Obviously the lower two points are around the sides of the rear seats. I fitted a second hand set of inertia reels from a 964 C2, however the shelf is completely different on these cars and hence not suitable for the 3.2 . You can modify your own shelf by installing the plastic trims that form the belt webbing surround, Porsche Part numbers 964 555 511 0101C & 964 555 512 0101C.
I've not yet installted the trims, I'm trying to find a car with the parts fitted so that I can take measurements prior to going in with a knife and cutting the shelf !!
Good luck,
