Unfortunately, Z cars have terrible aerodynamics. The 240s are like .46 cd. Even with a g-nose they're still in .40 territory. Its a shame, because they're beautiful cars, with an engine bay that can accommodate just about anything you can think up.
Don't be afraid of turbos, even though they make more power, they actually get better economy too (in general)....as long as you stay off boost. Plus, you can use a physically smaller engine without sacrificing enough giddyup to not be driving a slug. Something like a suzuki 3cyl turbo would be a good choice....they only weigh like 150 lbs! The stock l24 is like 500lbs, so thats a gigantic weight savings.
My second vote would be TDI....probably the only way you're going to see 40mpg or higher in that car. I don't know if this has been mentioned or not, but nissan made a diesel version of the inline 6. Its a LD28, I think? It would be a pretty much bolt-in conversion. No weight savings there, though.
Have you considered electric? I originally was going to do the 350/LS1 swap into my 240 but went electric and haven't looked back. You can commute on pennies and still burn rubber. I'm on my 2nd electric Z project now....if you have the skills to swap engines you have the skills to drop in an electric motor. Really depends on your range requirements if it would be practical. Plus you can do so much more with the aero....like a complete grill block, full underbody tray without exhaust getting in the way, etc.
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