You can swap heads, but you'll also need matching electronics (02 sensor, ecu) and you'll be a bit off, as the vtec-e block from the d15z1 is a 1.5 with different compression pistons.
As such, you likely won't get the optimial fuel mileage as you'll be running the wrong 'base' program for that compression and thus different horsepower availablility.
honestly, I'd just throw in another z6. They are a dime a dozen and cheap. By the time you factor out your headaches of wiring, locating, shipping, installing, etc, it will probably tkae you 3 years of driving to break even on the gas savings.