The Smiths guage is negative ground, but I've no idea how to find the specs on it, I've heard that older cars usually use 0-30 ohms...

VDO guages are sold in models with these various specs:
0-90 Ohms
10-180 Ohms
240-33 Ohms
73-10 Ohms
10-80 Ohms
90-0 Ohms Tube Type Sender

The 300D manual says the sender goes from 1.6 (+/- 0.7) ohms on Full, to 56.9 (+/- 1.9) ohms on Empty. So, does that make the sender a 60-0? 0-60?

Maybe if I look at the VDO website I can determine if "0-90" is for empty-full or full-empty...

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'82 300D Turbo 145,367mi