Extremely late update, yet, wanted to say I went with the follow process:
I ended up purchasing a "3 Point Retractable Seatbelt with Genuine Licensed GM Seat Belt Buckle" from Wesco Performance.
https://www.wescoperformance.com/3point-seatbelt-gm-retr.htmlCost me $110 in august of 2017, with tax and shipping for one seat-belt.
The re-tractor box fit nicely behind the bench as you can see in this video.
https://youtu.be/VB0plIkgQxAI went under the truck to drill a hole for the installation of the re-tractor box as to ensure I didn't drill into the fuel tank from above.
I was relieved to see that I had ample room to work in drilling this hole. Afterwards, I used the nuts and bolts provided from Wesco to secure the re-tractor box.
(I do not remember the drill bit size used)
Another note is I went with the 11'' bucket seat sleeve.
20'' would have had my receiving end of the seat-belt almost towards the edge of the bench, but 11'' was really a nice fit in my opinion.
I had called Wesco to insure that I can remove the plastic sleeve intended for a bucket seat, and sure enough, I was easily able to do so.
https://youtu.be/SoOSwz6Ds6AThe only 2 downfalls...
The bolts provided from Wesco were not fitting the existing seatbelt holes in the truck. It was super close, but the threads were thinner on Wesco than need be.
I has used two of the remaining bolts from the original seat belts on the floor bed, which allowed me to properly secure both the 11" receiving end and the shoulder mount.
The other downfall was that, the original bolt that I had used from the truck to secure the shoulder mount had too thick of a head on it.
Thus, the plastic cover that hides the bolt cracked when I tried to place it.
I'll be calling Wesco soon to see if I can order proper threaded bolts for a 73' C20 and a replacement cap for the shoulder mount.
If so, I'll be ordering a new one for the passenger side as the seatbelt itself is great and looks sharp.