For the weight of the 240sx vs. the 300zx and the power difference from stock, the 300zx brakes are more then capable even at higher horse power motors... well depending how much weight is added to car.
So yes the answer is yes. The easy route is swap out the Brake Master Cylinder with one of the 300zx ones, new front rotors/calipers/pads/hardware and SPL brake lines, done! oooooooooooh one problem I've seen there are 2 different kinds of SE rims (they look the same from the out side), one will rub and one will not at least thats what I've read, I have no idea about the steelies. Best bet get some wheel spacers (or just pull off the old cali and slap on the new and see if it touches before you decide), I recommend front and rears other wise it makes the rear feel lazy and pain to drive on the highway. Now on size.... Not sure, brake out that ruler! uuuum oh yeah the master cylinder... damn it!!! look here!
http://importnut.net/300zxbrakeswap.htm#frontbrake