Wiring an Inverter into a Car
You are best served with short thick input cabling.  

You may also find that having just one battery will cause you to rethink your friends kind gift when you run your battery flat in 20 minutes.  Perhaps less than an hour with the engine at cruising revs. If you decide to add a second battery you can mount it close tot he inverter solving your cabling concerns a lot easier.  Make sure you arrange a suitable charging system to isolate the starter battery if the engine is not running.

The peak current may be around 350A, that is welding currents and will need welding cables.

If you only plan to use it occasionally, you could add carry handles and then fit nice traction battery power connectors (the sort used in electric forklift trucks) and lug the unit to the battery when needed.