The 250SL had 82.00mm standard pistons which are probably available in .5mm and 1.0mm oversize maybe more. The 280SL had 86.5mm standard size and probably should only go one oversize because of the narrow area between some of the cylinders.
You probably can not bore the 82mm block to the size of the 280. You will have to go over size on which ever motor you have.
Bosch Master, ASE Master, L1
33 years MB technician