When we imported our 280SL from California it was still fitted with the origonal Kangol seat belts, whilst these were still in excellent condition to comply with Australia Car Design Rules, we were required to fit retractables
As you can see from the attached photo we used a short lenght of 50 x 50 x 5 mm angle iron with two 10 mm bolts high tensile trough the floor with a 50 x 5 mm flat plate under the floor the new belt drom is bolted to the vertical side of the angle iron. Along with the drums this is mounted under the removable panels behind the seats hard up against each side, completly out of sight where they carn't be damaged by other things.
We pulled back the carpet on the floor cover boards and shaved approx 4 mm off the wooden boards to allow clearence on the belts, we did not alter the carpets. We have now also fitted belt holders to our head rest support rods, saves fidling around trying to find the bealts once your in the car