Harey on here has a tow ball on his GTT I believe.
I have to agree with the above though, OP. A Skyline is really the wrong car for the job, they don't have the low-down torque needed to pull a trailer effectively. RB's love to rev, they're not designed for pulling power. You also won't get anywhere near 10l/100km's with an untuned RB25DE. My old engine used to get me about 350km's to a tank. Kwaka is spot on, an AU or even an older EL/EF Falcon would be more the vehicle to suit you.