If you want install grub to the MBR, you can run grub as root. Then run the following command in the grub shell:
grub> root (hd0, 5)
grub> setup (hd0)
where (hd0, 5) is the partion your grub is installed. (hd0,5) means /dev/hda6.
Changing grub's other configuration doesn't need to access MBR.