Yes, if the new disk is the exact same as the existing (but check the number of LBA's as even disks of the same model number can be different sizes) then you could use 'dd' specifying the raw disk 'rdsk' device to copy the whole thing.
However, in my experience, if you want to clone a Solaris 8 system (ufs filesystems) then you might just as well do a disaster recovery (DR) procedure straight onto the second box.
See my post#6 on this thread:
Restoring a system from a backup
for detailed steps.
This, of course, assumes that you are backing up your production box using fssnap/ufsdump to either a tape drive or a NFS remote disk.
Remember that when the restore is complete NOT to boot the second machine on the same network because the node name and the ip address will be identical. You will need to modify those first.
Also, read jlliagre's post#7 which is highly relevant and another method to do it.