The Whitney unit has been very highly recommended on the mercedes-veterans.com board.
You can also get the carbs very close with a length of heater hose or an automotive stethoscope, listening to the noise at the base of each carb and adjusting the idle accordingly.
E-mail me if you need the adjustment procedure off the CD.
I have run unleaded in my coupes for years with no problems.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe