The 280's also have a 2.8L instead of a 2.4L. Now, if you want to get fancy, you could find a crank out of a Maxima diesel and put it in a 2.8L. this would give you a 3.0L L motor. I think they were called a LD28 and only came in the Maximas.

BTW the Maxima gas engine is a 2.4L with the oil pickup in the front. The boss is there if you want to drill it for a rear pickup.

If you put a E31 head (early 240) on a 2.8 you will have a high compression engine with stock parts. You should at least put the 280 valves in so it will breath.

Oh and the 73 240 and 74 260 both have the square Hatchi carbs that aren't worth a crap. That is why the 280 went to EFI. The EFI on these cars is very simple and can easily be tweaked.