In this photo we can see the oil breather port of the external oil tank peeking through the large hole https://pbase.com/9146gt/image/159636988 and the possible obstruction of the oil breather line by the left side of the oil-thermostat body https://pbase.com/9146gt/image/159639237. One of the tricks here is to connect the three oil lines to the oil thermostat first, then bolt the oil thermostat to the mounting bracket welded in place.

There are other considerations too, the oil line connected to the oil thermostat at the right-side, the 90 degree elbow of the oil line could be hitting the firewall, see this photo https://pbase.com/9146gt/image/159636950