hello Andrew i had a similliar problem on my big block cobra tried everything,
shrouds/ fans/ e waterpump/
oil cooler each time cost money car never boiled
but i new something was not right took me 5 years to figure out .
dont drive the car very often and yep engine builder put head gasket on backwards,
i bought new set of gaskets then sat them on top of rocker covers and looked where
the tags were proud around outside of the head compared to the new ones sitting on top and you could tell without pulling the heads that passenger side was wrong now it never gets hot at all . so try and see before pulling heads it was really easy to spot in the end on the bib block anyway.
chris