You must remeber valid keys for any version of Houdini are also valid for every previous version!!!
The best thing is first install 6.0.304 version of Houdini with license manager, use included crack (copy included in houdini 6.0.304 sesinetd and replace existing /usr/lib/sesi/sesinetd.
Use included keygen for v 8.2 (run it on Windows machine), copy keys to license management window in your IRIX machine and start server.
All previous versions of Houdini should run (6.5.51 and 6.5.67 too)