The manufacturer specifies battery for one reason. They want a clean well fed source. After seeing the way many cars are wired I can see why. Does it have to be at the battery, no. You need a tight well thought out feed source. Where will those points be with the original harness? Battery, starter relay main stud, starter main stud, alternator main stud, ampmeter studs. First thing I would do is run a 2nd circuit between the alternator output stud and the starter relay (8g). Now if all terminals and wires are in nice shape you could pull from starter stud, inconvenient. Battery, cluttered. Alternator, long run across the engine. Starter relay std, best spot. Battery in the trunk is another story. This is best served by installing a bulkhead in the firewall. Run the battery cable to the bulkhead.On the engine compartment side you can attach the starter, alt, etc. Inside you have a nice 3/8" brass stud to pick-up power. Speedway has a nice one.
http://www.speedwaymotors.com/Battery-Cable-Bulkhead-Connector,2353.html.
Here's mine on the firewall.
Doug