I find it confusing your (turbo!) cars seem so slow. My '78 NA (4 speed, 3.46 rear end) did 100mph and the only thing stopping it from more was the governor

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1978 300D, 373,000km 617.912, 711.113 5 speed, 7.5mm superpump, HX30W turbo...many, many years in the making....
1977 280> 300D - 500,000km+ (to be sold...)
1984 240TD>300TD 121,000 miles, *gone*
1977 250 parts car
1988 Toyota Corona 2.0D *gone*
1975 FJ45>HJ45
1981 200>240D (to be sold...)
1999 Hyundai Lantra 1.6 *gone*
1980s Lansing Bagnall FOER 5.2 Forklift (the Mk2 engine hoist)
2001 Holden Rodeo 4JB1T 2WD

You're going to need new injector nozzles, get the cores sent out first since there's some turnaround time.
Check timing chain stretch, correct or replace.
Advance your pump timing until it starts to knock at full load, then dial it back a degree or two. You will end up around 28*
Shim or remove the alda.
Add a boost controller and set it at 13-14psi.
Set the transmission modulator pressure correctly or it will be slipping.

You can yank the power steering pump, just hook the steering box lines to a tee with a vent. I ran my w123 like that for a year or so with no trouble.

Cheapo jeep 1" spacers can go under the rear coils. Yes it will mess with your camber, but it will get you the boost you need carrying that much fuel

edit: Just saw your update. It definitely wasn't tuned correctly, a w123/w126 that's up to snuff should be able to hit 110 very easily.

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$60 OM617 Blank Exhaust Flanges
$110 OM606 Blank Exhaust Flanges
No merc at the moment

You are right; it should hit 100mph. Just found this spec for my '81 SD:

With a maximum top speed of 103 mph (165 km/h), a curb weight of 3571 lbs (1620 kgs), the W126 300 SD has a turbocharged In-line 5 cylinder engine, Diesel motor, with the engine code OM617.
This engine produces a maximum power of 122 PS (120 bhp - 90 kW) at 4350 rpm and a maximum torque of 250 Nm (184 lb.ft) at 2400 rpm. The power is transmitted to the road by the rear wheel drive (RWD) with a 4 speed Manual gearbox.
About chassis details responsible for road holding, handling behaviour and ride confort, the W126 Stock tire sizes are 195 / 70 on 14 inch rims at the front, and 195 / 70 on 14 inch rims at the rear. For stopping power, the W126 300 SD braking system includes Discs at the front and Discs at the rear.
The W126 model is a car manufactured by Mercedes Benz, with 4 doors and 5 seats, sold new from year 1981 to 1985, and available after that as a used car.
read more at: https://www.ultimatespecs.com/car-specs/Mercedes-Benz/279/Mercedes-Benz-W126-300-SD.html