i agree with bartamos,
any seat belt we think we will install in an old truck may end up doing harm and it might lead you into a false sense of safety
why not leave the home made seat belt out as a constant reminder to the occupants not to get fancy with the driving
be safe and defensive
for a good read, the minimum standard the law requires for seatbelts can be found here:
https://www.ecfr.gov/go to Title 49 → Subtitle B → Chapter V → Part 571 → Subpart B → §571.209
Title 49: Transportation
PART 571–FEDERAL MOTOR VEHICLE SAFETY STANDARDS
Subpart B–Federal Motor Vehicle Safety Standards
§571.209   Standard No. 209; Seat belt assemblies.
-s
