Also just to make sure your mounts are like this:
/ = sda5
/boot = sda6
And you are mounting that like this, during grub-install
/mnt = sda5
/mnt/boot = sda6
If that is the case...
This will install the boot loader into the mbr:
grub-install --root-directory=/mnt /dev/sda
You may need "--recheck" additionally...
Otherwise, I'm not sure.