Thanks for all the responses. Both Tremec and Richmond gear use the "fill till it spills" method, no reference to capacity, so that's how I'll go. No reference to the 717.000 tranny in Benz's "specifications for operating fluids" so Ill use the 75/80 gear oil recommended for the early .400 not the later ATF.
https://bevo.mercedes-benz.com/d/d/en/Spec_231_1.pdf