Did you try to fit an o-ring? Lots of thermostats will come with an o-ring, it should not be used here. When you fit the thermostat into its cover and try to fit it all onto the housing it should fit flush with the housing with barely any gap. When tightening it all up, if you feel its getting tighter and there is still a gap between cover and housing, stop immediately, there is something wrong and you should undo and check everything again.
Basically any force on each side of the cover when its not flush with the housing and you'll break it.
I've broke two myself over the years, these days when changing thermostat I get to it like to a newly born child
1997, 4.0 LWB X300 - Unlucky car but still a daily driver.
1988, 3.6S XJS - Never ending restoration project.
1994, 3.2S X300 - Accident damaged, October 2015. Gone!
1994, 3.2S XJ40 - Long gone.