Mate I've installed two coolers and it's a good 2-3 hours work to do it well. One was a Greddy and the other was an ARC. Both we kits and both were a major pain the arse, the ARC was lesser though. Although you buy a kit the instructions are never in English and they never fit perfect.
In my job I charge $120 per hour. If you cant afford $250 to fit a cooler then I'd think twice about driving a skyline because if you bend it or need to mend it, you'd better be prepared to say goodbye:)
Think about it this way, drop the car off, do your own thing, pick it up - coolers done......mmmmm relaxing.