.../...clutch master sleeve for a 250 rather than a 280.../...
The clutch hydraulic system is different on a 250SL compared to a 280SL.
This is due to the change over to a diaphragm type clutch mechanism on the M130 engine.
The dia for the master and the slave cylinders should be a match to each other, as follows:
230/250 SL clutch master cylinders share the same bore (Ø23.81mm) but come with different part numbers. The slave cylinder is the same for both (Ø25.4mm).
280SL master cylinder is Ø19.05mm bore and slave cylinder Ø23.81mm.
It is not trivial, and as DanC mentions, complicated by use of M12 or M14 fine pitch thread pipe connections. The SLS website has a useful table; see Section 29.i