a 300D and two 300SD(s) and not had a problems. These are all 14-in and so I would think that so long as you are changing 14-in for 14-in and you don't mix the offset of the wheels that are mounted on the ground, you should be OK... with one possible thing I don't think was mentioned. Did you say what size the good tires are that you want to use? I ask because many W123 chassis models will have smaller 195x70x14 size [not sure of these #s] where as the 300SD(s) have a slightly larger tire... 205x75x14 tires. Again, I'm entire sure of the #s because I'm on the road now away from these cars.

I would not put the smaller tires on the larger, heavier 300SD W126 car.
Sam