How to use MOSFET thermal data for simulating in software
The IRFB7434 is a rather modern MOSFET, possibly employing trench gate structures. In linear operation, these modern MOSFETs can exhibit a positive temperature coefficient on a local scale, leading to current localization followed by local overheating and finally by failure. Further information on these effects can be found in the application note "Automotive MOSFETs in Linear Applications: Thermal Instability" by Infineon.

All SOA diagrams I have encountered so far in datasheets do not take this effect into account and are thus invalid for describing linear operation behavior. This localization effect also renders thermal models invalid if these are based on the thermal impedance graph, as a homogeneous power loss density within the silicon die is assumed.

It is important to note that the SOA data is not actually measured, it is calculated from models and statistical data. Apparently, the models used by most semiconductor manufacturers are somewhat outdated. 

Long story short: You cannot use the SOA diagram of modern MOSFETs or the thermal impedance diagram to predict their behavior in linear operation. Maybe you can modify your circuit to prevent linear operation.