LDOM (Oracle VM server for SPARC), with HA LDOM solaris cluster between physical nodes, it will cost you since SC is 'per core'.
Trouble is : If a node fails for some reason, cluster will only boot the LDOM on the next host (couple of minutes of downtime, but downtime).
You can migrate old machines to future LDOMs using flar archive or similar method.
Option is using LDOMs and running Solaris cluster between virtual machines. Depending on your resource (application) you will be able to configure active / active or active / passive.
If a node dies, other node and its ldom with SC will takeover, while you can just import the rest of the machines (ldoms) to working node (what SC HA LDOM does, but automatically, in above example).
Be sure to have auto-boot?=false for all ldom's which are on shared storage.
You don't want to boot root zpool on two nodes at once
You might also use bare metal with branded zones (doubtful solaris 8 will work that way) and Solaris cluster with HA ZONE or Solaris cluster zone cluster for high availability.
Or just importing the zone manually if the node dies, since zone rpool will not import on another node if configured properly (it will give error that it is active on another node).
Hope that helps
Regards
Peasant.