Hello Brutus,
CH/RH-53D's and the early CH-53E model had the troop seats that you have installed. We would wind (a single strand) green parachute cord from the bottom of the fwd most window -port side (called "gunner's window/1st mech window) then aft, usually just wrapping around the next meat hooks (litter mounts) and continueing aft to the ramp. Another Parachute cord was started at the aft end of the crew door (crewchief's side-starboard) at the same level--the bottom of the fuselage(cabin) windows and run aft, all the way till the ramp rib. The troop seat-belts were strung over the parachute cord to help on-boarding troops to visulize their "butt-parking" area, and then only made it look somewhat organized.
In the side fuselage pic which shows your lap belts: looking right to left(fwd/aft) the 1st and 2nd belts are about correct....each troop had a left and right belt which were connected to a ring mounted at the top(horizontally) OF THE TROOP SEAT, we would then run each left/right seat belt-half up and over the parachute cord, connect the seat belt halves which made a somewhat triangle/pyramaid shape. Between windows (between meat hooks) the cord would be streched lower in the center due to the tightening of the seat belts to reduce slack/flapping.
The left-and-right of.. center fuselage floor..squares were dual purpose: 1.) each square housed a receptacle to recieve each..vertical..troop seat leg, and 2.)provide tie-down up to 4400 lbs per. ...CH/RH. I don't know the tie-dwon value for the "E".
hope that helped, Brian