if you are looking for good power and fuel economy, stay with the six, and modify it. there are new performance parts available for the small six, including cams ground specifically for the six, one new aluminum head on the market now, and a new head that will be better coming out soon. you want efi? efi you got, for the stock head six, build it like offenhauser did with theri 3x1bbl carb set up, and then bolt on 3 tempo efi throttle bodies, and use a megasquirt efi controller, with or without the ignition control.
www.fordsixparts.com has the cams and headers as well as other parts for the small six. and by the way, these little sixes are tough motors, you almost need a nuclear warhead to kill one of these things.
oh and if you want a turbo or supercharger, they can be set up on the six also. if you want serious power, think about this, use the new fspp head with it comes out, along with an fspp cam, what ever efi system you want to use(efi is easy to adapt to any engine), and a pair of the small ihi turbos from an 87 tbird turbo coupe. run the boost up around 20psi, and you should be in the 400hp range.