Sorry, I can't help with the cable retainer since I wasn't worried about the wiring during my one experience trying to get a spare pump at the pick-n-pull. I just had a ton of trouble getting the sender unit to have enough room to back out of the hole (this was in a sedan, wagans are supposed to be easier).
But my recollection of the bung nut was that it was a big, plastic "female" ring and easy to turn without any special tools. Not at all like the recessed, metal "male" bung nut on the 240s (and maybe earlier 740s?).

The following idea must have a big flaw because I have never heard it discussed on this very inteligent board. When your one pump dies in a Rex-Regina car, why not just add a pump in the line outside the tank (like the LH cars have) ?? I might have thought the dead pump would impede the flow, but my recent experience adding a (Volvo) windshield washer fluid pump in the fluid line above the dead one in the resevoir of my Town and Country tells me that the dead pumps really aren't an impediment. Please shoot me down on this.
--
1991 745 213K miles, 1991 745 210K, 1990 744 GL 183K, all Rex-Regina - past 240s