I am living through a nightmare with a 1999 DeVille. Bought it about a month ago and the seller did me dirty. He changed out the liquids to hide evidence of blown head gasket. I used Thermogasket and it appears to be working though the car does still get up to 222 degrees in traffic-time idle. Not sure why. The temp comes back to between 195 and 206 at highway speed.
Today makes the third time I have run out of gas because the gas gauge tells me the truth only some of the time. When I fill it up it tells me its full and goes down when I deplete the fuel. When it drops below 8 gallons I don't know what's in the tank because sometimes it tells the truth, sometimes it doesn't. I suspect it's the sender unit based on what I've read in these forums.
I have a 1995 DeVille that has problems I am unwilling to fix. I am wondering if the sending unit from that one will work in the '99.
The '99 is the 4.6 Northstar. The '95 is the 4.9 PFI.

Anyone have any advice?

By the way, the PFI engine and transmission seem to both be working fine. The rest of the car is basically parts. Some of it works, some of it doesn't. I successfully replaced a door window motor in the '99 from the '95. I got the car for $425 from a title loan company who repo'd it from someone who had to have been a drug addict based on the condition it was in when I got it. I've done all I am willing to do with it.