Other battery relocation things to consider.
Increase the size of your positive cable by a size gauge or more over stock. Length=resistance, so you may not have enough power left by the time you reach the starter, or you may super heat your main cable. Both can be avoided with larger wire.
Install a heavy duty breaker at the battery. Since you will now have a large "hot" cable running the length of the car, you have increased possibility of that cable shorting to ground. A breaker will blow should this occur, preventing major damage or potential fire. Match the breaker to the maximum amperage potential of your positive cable. Both should be big enough to easily handle all the electrical demands of the car.
Here's a diagram I designed for the battery relocation (and addition of a fuse block) in to my 510 wagon, in case it helps.
Still working on wiring. The engine bay is getting closer to finished, but the dash is still a complete joke. And I haven't even started building a stereo harness. Baby steps, right?
But I drew these up last night. I have always hated the fusible link setup in my 720s, and now the same one is in my wagon. I think I've come up with an excellent upgrade to that design, but may be wrong, so feedback is appreciated.
Relocate battery under rear seat.
Install high amp breaker right next to battery. If long battery cable ever shorts to ground, it trips, hopefully preventing any real damage anywhere. Breaker also has manual switch, so I will use it as a kill switch.
Run cable up to engine bay.
Install power distribution block on shelf where OE fuse block sat. Batt cable splits in two directions. One directly down to starter. One to maxi fuse block.
Install maxi fuse block on OE fuse block shelf. Wire car harness directly from that box in place of fusible links.
Now I just need to figure out the proper amperages for the fuses.