Actually, I'm getting 12 to 13 mpg with my side oiler with 428 crank and dual four barrels. Special built motor for street, small carb's, good manners, custom pistons for quench and a vacuum advance dist. Still, I run the 1/4 at 11.9 on street tires and a close ratio top loader (close ratio is a poor choice for drag racing), so it's not slow.
Thats steady freeway cruising at 70 mph and no screwing around. Around town? Screwing around? 8-9 mpg
If I was building a new one I'd have to consider a 428 with single carb and a roller hydraulic cam. Again, I'd go with custom pistons to get some quench. Even a 427 center oiler will save some bucks over the side oiler and still be perfectly fine.
My top loader was needing some work so it was either major rebuild or go with a TKO. So I went with the TKO600, I'm pretty sure my 1/4 mile ET will drop now that I have some decent gear ratios to work with. Kind of nice having that lower cruise rpm as well. To early to tell if it's doing better on mpg or not. I suppose it will, but what ever...