> Are there any way I can figure out the position of one of the backup super-blocks?

"Block 32 is usually an alternative super block."
		fsck_ffs man page

If that fails you can hunt for the superblock magic number.

Be aware that running fsck on a filesystem with read errors can
make things worse.  (Give you one guess how I know that.)

Safest thing to do is copy the filesystem somewhere, skipping
over the blocks that are unreadable, then run fsck on the copy
and see what happens.

See also fsdb(8).