The reason for "data block too large" error is that the program donīt care about the extras pointers so think the first block begin 0x8000 byte before, so the block is larger than 4096 bytes.
user.fs.v5 file has a jffs2 filesystem, so is not a cramfs.
To mount a jffs2 (see http://www.esubuntu.com/how_to_mount_a_jffs2_image
that refer to http://alanren.com/archives/12
) you can
$ mknod /tmp/mtdblock0 b 31 0
$ modprobe mtdblock
$ modprobe mtdram total_size=65536 erase_size=256
$ modprobe jffs2
$ dd if=/path_to_image/image_name of=/tmp/mtdblock0
$ mkdir /media/jffs2
$ mount -t jffs2 /tmp/mtdblock0 /media/jffs2
But I think the user.fs.v5 file is useless. Libraries and exec are in root.fs.v5