whats the best engine for both performance and mileage??

I found an almost rust free 70 dart and am trying to do a cost effective mild restoration. Planning an overdrive automatic and would like good power. I was thinking about stroking a 360 but would I be better off with a 318. I can get the 318 engine and trans out of a 91 pickup for nothing.
Any suggestions and what kind of MPG can I get with various senerios. PS I was planning a 3.55 Posi.
Thanks
Greg Wise