A light that ways upwards of 150 pounds is NOT safely secured to a box. Even fan mounting boxes top out at 50 (though the code limits fans to 35 pounds). You are going to need to make some other structural support arrangement to hold this monstrosity.

Heavy devices can be supported by a cable that is run from the chain, through the bottom of the junction box and up to a support structure. I've used a 2x4 that spans a couple of ceiling joists, plus appropriate hardware.

Thank you all for the replies and advice. It seems that the fixture is actually around 60lbs not 150lbs, that was the total shipping weight so should be a bit better to install!

Still 60 lbs exceeds the capability of most fixture boxes.

There was 90lbs of stuff in the box that isn't part of the fixture? Was it in a wooden crate or something?

try this

Here is a link that might be useful: fan brace

"There was 90lbs of stuff in the box that isn't part of the fixture? Was it in a wooden crate or something?"

Maybe it was a refugee