There is one thing, when it comes to swapping seat belts (or other parts also) that you will run into if you haven't already and that is the fact that over the years srs made significant changes in the way the belts engage in the receptacles.
I learned this the hard way after I swapped some new seats into my car. I bought some pristine seats from an '82 300D to replace some in a '83 which had some worn spots. After I put them in I discovered the belt ends did not fit into the receptacles on the seats! I mean not even a little, they wouldn't fit AT ALL! So I had to remove the front seats to fit the old receptacles, they are on a long stalk that slides back and forth as the seat moves that a ways. Nothing is worse than being real tired after a lot of wasted effort to find it has to be done all over again!

__________________
'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!