The *floating feeling* is a function of the progressive action dampers(shoks) the new cars have, for small undulations, there is minimal damping to give a smooth ride, but for large amplitudes(movements) the damping is very much stiffer. I have driven a E230 W210 at over 160kmh on the north-south highway and the ONLY criticism I have is the power steering is a bit too light weight for my tastes.
I doubt that any aftermarket foam can make a W126 chassis any stiffer, the car is already built like a battleship !
The foam injection into the a-b-c pillars has been done by manufacturers for years as it insulates wind and reflected road noise, my brother's Audi 200 Turbo had foam in all the body pillars and around the *doubled* firewall.
One last thing I should mention, I have seen a trend of using 16in and even bigger wheels and tyres on the W126s around here, be aware that the wheels have to be for a W126 as the offset for the Mercedes front strut suspension cars is not correct for a 126 and will caused premature wheel bearing failure.
Have a good week.