I have both a W126 and an R107. The 300SEL only outweighs the 560SL by less than 100 lbs.
I'll check my parts reference for sure, but I don't think the front brakes are interchangeable. The rears are. It also makes a huge difference if you are talking pre- or post-1986. I know the rears use the same rotors and pads.
I believe that the 560SL only used ATE calipers. The W126 used either ATE or Bendix. But even the ATE is not the same caliper. They both have the same wheel clearance, so physically, it may be possible to swap parts. I guess that just depends on the caliper mounts. I've never compared the front rotors, so I don't know if there's an actual difference in the hub size or diameter of the rotor, although I suspect the later is unlikely.
I don't think the master cylinder makes a difference in the calipers because my 300SEL has a Girling master cylinder and ATE rear calipers. Haven't done the front brakes yet, so I don't know what brand calipers are up there.
1988 560SL Black/Palomino
1988 300SEL Black Pearl/Burgandy
1984 500SEC Anthracite Grey/Palomino