Scott if you are in Adelaide.
1. The last time I checked you don't
have to fit seatbelts to a car so old, if you choose to do so no inspection is required. It would pay to check with Regency though.
2. You can get belts elsewhere, but Andrew is pretty good with his pricing at Rares.
3. You can get pre-made anchors (plate with a nut welded to it) for a few dollars ($2-3 last time I bought some) each from Willshire Motor Trimmers on South Rd. Saves stuffing around and is legal spec.
4. B Pillar installation.
Using the plates from Willshire I cut a slot just big enough to slide the plate in on the B pillar, the slot needs to be just above the top hinge of the rear door or you will have trouble getting the plate up the pillar. Then drilled two holes further up, one for the bolt to pass through and a smaller one just above to use for a guide wire.
Feed the guide wire (MIG wire works well) down the B pillar to the slot, attach to the plate via hole that you have drilled. Fuss around for anything up to an hour to the plate in position and secure in place with the seatbelt bolt. Remove the guide wire and pop-rivet the plate in place.
I covered the slot with a piece of alloy that I polished up and screwed in with polished stainless screws, you can just see it hiding behind the belt in the pic.
As for the rear belts, I put the top mounts on the parcel shelf.