I was very happy with the inertia reel kits that I fitted on my rear seat. Personally, I would not fit used belts and get new items for safety reasons. I got them from Canley Classics:
Securon Part Number 254 £38.60 inc VAT each.
http://www.canleyclassics.com/searchforapart/?desc=securon&ptno=254
Don't be put off by the fact that it lists them as for a Triumph Dolomite or 2000.
Just looked at the link on the other thread Rich points at and the eBay trader is doing them for only £26.96 ea.
As Rich says, the anchor points are already there as rear seats belts were an option on P6s. Each seat belt has 3 anchor points which are 7/16" nuts welded onto the chassis. If you lift out the rear seat squab and remove the seat back, you will find the two lower points (near wheel arch and near centre of back seat). To get to the third one for mounting the interia rear, you need to remove the parcel shelf.
If no belts have been fitted previously, you need to drill a 7/16" hole in the parcel shelf (through the PVC covering, hardboard and sound deadening) to reach the mounting nuts. I also used a number of 7/16" washers stacked up between the interia reel and the nut on the chassis. This was to raise the inertia reel enough to stop it being pulled down into the soft sound deadening material. Apart from drilling through the parcel shelf and making sure you get the holes in the right place it's a really easy job. The Securon kit also came with instructions which helped. Before I removed the parcel shelf, I marked where I needed to drill the holes in the parcel shelf by sticking a felt-tipped pen through the captive nut which can be accessed inside the boot area.
The securon kit comes with plain anodised mounting bolts. IIRC at least one other forum member got some nice stainless steel ones which maybe look a bit nicer.
