The belts shown have open clips so they can be attached and easily removed. You don't need any special fixings but they wuill need to be connected tio eye bolts. That will enable the belts to be left in-situ or taken out when not needed. There are, however, some regualtions on eye-bolts.
Eye bolts for the securing of harnesses (as I seem to recall the DVLAS (uk) saying) need to be secured to a captive nut. For the lap belt this means you can remove one of the bolts on the door side of the floor mounting and replace it with en eye-bolt. For the centre you will need to attach it by replacing the bolt that hold the femaile end of the seat-belt to the car/seat.
For the shoulder harnesses these will need to be attached roughly at the rear of the lower swab of the rear seat. In front of the rear seats and the angle is too acute. Above the rear seats (i.e. the parcel shelf) is not regarded as structural and will not be permitted. You can not simply attach the eye bolts through a hole in the brace member with a nut. Instead yyou will need to secure a captive nut first and then aattach through a hole to the same captive nut.
The R33 I looked at had them in as well as the standared belts I was told that under the rear seat the Captive bolts are already there for the Long back staps so sould not be any Problems and then you Take the Rear bolt from the Inside Seat runner and put in the Eye bolts for the side strap and the other one fits to the Bottom of the Main belt neer the door by taking out the bolt
I was told that fitting should not take any longer than hour for both sides with no holes to drill
That was each. They do a lot of different makes in the Demon Tweeks catalogue, Sabelt, Momo and saftey devices. They come in different belt widths to. The Willans 4 point belt (same as the Nismo) is FIA approved and one of the more expensive ones but all are obviously guaranteed.
They are made by Willans and Nismo just sticks the name on them. The price is for 2 but I really prefer a wider strap myself. Willans do make different models and the only reason I'm cosidering these is because most people always go for Sabelt. Plus they are used in Japanese racing so I feel thst's more "approproate"