Ive done it before with a rb26 in a car that had alot wider engine bay than a gtr so it was a pain in the ass but still relitively easy(than again im alot more mechanically minded than most).. if you have access to a hoist were you can simply drop the whole lot out from underneath and have it sitting on the ground while the car stays up in the air.. you could get it done in a solid day or 2 however it would be an absolute nightmare to do it while the motor is still in the engine bay if your confident attempt it.. if your hessitating get someone else to do it or just wait till the headgasket blows before you install a metal head gasket, the standard items can put up with quite alot of punishment before they let go.