Your latest controlfile should have the latest information. RMAN will take the information from the controlfile.
If you are restoring the very old controlfile which does not have current information, than in latest releases you can "catalog backuppiece 'backuppice_name' which tells RMAN about the backup BUT I don't know if you can use this command in 8i. Try it, If not than you have to find the way to tell RMAn about this backuppiece.
But if you are retoring the latest controlfile, it should have that information about the backupp iece. Try it, you have nothing to loose. Do it to beleive it.