30mpg might be tough. The 2.4 seems like it would be the better option for getting that to happen. You'd also have to try and keep the car as light as reasonably possible.
auto trans only if you do a lockup converter. otherwise stick with the 4 speed.
smaller motors will get better mileage but you should still get over 20mpg with a properly tuned 318. but again with a V8 you are bringing a lot of weight with you also.
