The way i look at it, if im gonna build something, im not gonna be looking for fuel economy. Keep the six in the skyline or if you are going to go 4cyl then definately an sr20 not a ca18. The silvia's boogy with these and with the r30 probably being lighter you could get one to up and go nicely! But definately an sr20!!
Then again, mine is not also that bad on fuel, is pretty crappy around town but once i hit the highway i get a good 500-550km to a tank. You just gotta drive the car off boost and you will be fine, as soon as you get that turbo working the fuel guage follows..