I don't know if it's in the notch in the tank, but this time when I removed the "ceiling ring on the pump assembly" and put it on the rim first, then I was able to start the threads (still using some force to hold it down, but not as much as I had been using), and was able to tighten the pump down (brass drift and BFH).

I watched a YouTube video by TRQ "How to Replace Fuel Pump Assembly 2000-2006 BMW X5" https://youtu.be/asqvlLFTAng, which was the first I saw placing the rubber seal on the rim of the tank. I had installed it before taking it outside for installation. Who knew that it would make such a difference?

The X started right up, no delay, and the fuel gauge needle went straight up to full (not past full as it usually does on a full tank). However, my tank isn't full, so I'm gonna need to reset something. The sender arm went straight in, with no interference, so I don't think there's a problem with it.

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01 BMW X5 E53,3.0i-5L40E, 7/13/01
topas-blau,Leder-grau,"resto-project car"

Here:
14 Lexus ES350,3.5L-U660E
09 HHR Panel,2.2L-4T45E
04 Chevy 2500HD,6.0L-4L80E
98 GMC Sierra 1500,5.7L-4L60E

Gone:
66 Chevelle Malibu 2dr ht.,327>441c.i.-TH350>PGlide/transbrake
08 Cobalt Coupe,2.2L-4T45E
69 & 75 C10s,350c.i.-TH350
86 S10,2.8L-700R4
73 Volvo 142,2.0L-MT4
72 & 73 VW SuperBeetles,1.6l-MT4
64 VW,1.2l-MT4
67 Dodge Monaco 500 2dr ht.,383c.i.-A727
56 Chevy 210 4dr,265c.i.-PGlide