I thought I'd update my progress so far on this thread.

The Motorcraft SW873B sending unit won't fit. There is a steel line running to the bottom of the power steering pump, and the sending unit hits it, and I can't get the threads to line up well enough to start them.

I found another sending unit, Motorcraft SW2433 and have ordered it. It is smaller than a SW873B, the only problem is that nobody stocks it, I should have it tomorrow.
1997 Grand Marquis GS - Died
Now have a 2003 Grand Marquis LS