I do not know what thermostat housing you have fitted but you need one of these:- https://www.morrisminorspares.com/cooli ... es-p829645
The stud arrangement of the housing appears to be incorrect. One of the correct stud holes in the head appears to be unoccupied. I would suggest that you remove the thermostat housing and gasket and post a photo of the thermostat opening in the head on here for further investigation/opinion.
Last edited by philthehill on Fri Jan 22, 2021 8:35 pm, edited 1 time in total.
Unfortunately that does not make it right.
Before you order a new thermostat housing remove the old housing & gasket and lets see what the head is like in the area of the thermostat housing.
There may be a broken stud lurking under that thermostat housing and that may be why it is as it is.