I have several light fixtures, 8 feet by 2.5 inches, brass cover for the base, candelabra base, 8 bulbs. I want to install them on the ceiling in my foyer (where I have a carousel collection). Printed on the metal base it says wall mount only. What do I have to do to safely mount them on the ceiling?

Show us a picture.

UL approves a fixture so it will operate safely as designed by the mfgr. Turning a wall unit 90 degrees voids the UL approval. Most likely, the bulbs will be directing heat to what is now the upper surface, possibly not insulated, and may overheat the ceiling.

If I insulate the base with fiberglass insulation like many ceiling fixtures are insulated, I would think that I would have erased the overheating problem. Right?
I would include a picture but don't know how to add it here.

The fixture is UL Listed as a wall mount, and nothing you do will change that.

I agree the wall mount is designed to withstand little heat where a ceiling mounted unit is designed to withstand much more. The base may not be the only issue with heat. Some wall mounted lights have plastic around the bulb socket and interior components because heat is not an issue and ceiling mounted units may use a ceramic or metallic material in place of plastic or actually use a plastic that is rated for higher temperatures. It could also be the wiring used in the light itself that isn't rated for high temperatures. If it is UL tested for use in wall mount only, then there is a reason it wasn't rated for wall and/or ceiling mount. They specifically put that warning on there for a reason. If you install it on the ceiling you may discover the same thing they did at the laboratory but not in a controlled environment and burn down the house! At 8 feet by 2.5 inches it sure sounds like a nice light fixture but not worth the risk. I'm trying to visualize what the light looks like but am having trouble doing so with those dimensions and a candelabra base. What type/size bulb does this light use? Any name brand and model I could search for to get a better idea of what you are talking about? Insullating the base would prevent some heat from entering the mounting box but may not alleviate the problem they encountered with the light at the lab when mounted in a ceiling configuration.

Wow, I'm glad I found this thread! I have a "wall mount" only fixture I am using to replace the ugly "hole in the ceiling with metal frame and plexiglass" that comes with a 60s house. The fixture is very cool, but has six outlets (all wired together, only one set from the house to wire). I can easily mount it on the wall, but was trying to avoid covering the ugly gape left when I remove the ceiling light. I think that is a lot better than burning down the house; and it should put out just as much light mounted on the wall (like most over than vanity). Thank you so much!