Well my reply is not specific to this but I was speaking in general.
If the PAM is not supporting the combination he is looking for then there is no point wasting time for options. I believe the newer versions of the kernels are compiled to run on higher versions of OS and DB even though they support older versions of OS and DB but the chances of encountering issues are more with the old versions.
According to PAM only 721 EXT version is supported and only with Oracle 11G for Solution Manager 7.1. The standard 7.21 release is not supported. This is strange.
Back to the stack file generation, I would select the kernel and the files which MOPZ is suggesting and get the stack file generated and start the upgrade. If the kernel is creating problems during the upgrade then I would try with 720 release kernel and see if that helps.
The stack file should be generated once the source system is ready for the upgrade.
Regards
RB