If you're sure that your SL is running too cold all the time -
71 C = 159.8 F
79 C = 174.2 F
87 C = 188.6 F
I don't have the Mercedes factory engine manual for the 280SE/SL but my 'Haynes' Mercedes 280S/SE/SL manual specifies an 87 C thermostat. I'm running a 79 C in my '72 250 (2.8 - M130 engine) but it's carbureted. Unless your SL has hot running problems in summer, I'd go with the 87 C.