I did the update on my 70E Targa and am very, very pleased with it. Really transforms the car from an entry/exit/driver comfort convenience standpoint compared to the early non-retractable belts. The new webbing of course looks nicer and is most likely safer/stronger too.
I used TRW/Repa parts obtained in a kit from PEP (
www.peparts.com). Not cheap at around $150/side but good OEM stuff. NEW...you could probably save money using boneyard parts but I view junkyard/ebay seat belts kind of like used underwear. You never know where it's been.
The mount points are there from 69- IIRC. The install on my car required zero modification to hard parts; the only thing was to put the slots for the belt pass-thrus in the upholstered rear quarter (side) panels. I was redoing the whole interior at the time anyway so this was a non-issue for me.
Pics of my setup are here (partway down the page):
http://www.nosubstitute.org/911e/interior