You are clearly not starting out in first gear.

If you start in L, you will force a first gear start. You should be quicker off the line and get a shift to second at 10-20 mph depending on throttle. L should hold the trans in second gear.
Shift to S, and you should go into third and hold there. Shift to D and you should go into fourth gear.

I have never tried to fix or adjust a modulator. The information is in the transmission manual, which is not on the CD and is hard to find. Mostly they are replaced because they are cheap compared to the labor to adjust or fix.

The 450 would be a challenge, but you seem to have a talent for this stuff. I would worry most about the gas turning to peanut butter in the tank and injection system, and the seals in the fuel system drying out. The climate control servo is probably frozen. If you do a search on Wayward Woman you can witness my trials in bringing a neglected 380SL back. They are fun cars to work on.

__________________
Chuck Taylor
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