MSD 6AL wiring question

Just installed all new wiring harnesses in my 68 340 Barracuda. The MSD 6AL wiring instructions show connecting the heavy red and heavy black (ground) wires to the battery. Trying to keep the installation "clean" and looking neat. Can I use the post on the starter relay for the 12v red wire or is it best to connect it to the battery post? As for the ground I have a ground strap on the rear passenger side head to the firewall. Is this sufficient to ground the black wire to?