Yea, I don't necesarly want the 2.4 liter for power, or displacement. I was considering the KA because they're simply everywhere, and I think once tuned well it will have more lean burn potential than the L series. The stock HP is fine, if not more than enough. Long runners with well placed injectors, megasquirt + EDIS I should be able to get that thing running insanely lean under the right conditions.
I'll only chime in to say that I have a 1971 Datsun 510 with a KA24DE swap including the 240SX's 5 speed transmission and no way do I come close to the 40-50 mpg you originally discussed in this thread's first post.
I didn't do the swap, it was done by McKinney motorsports in San Diego and the car runs great...but it's not tuned for mileage...just throwing it out there for what it's worth.