I'm getting an early 90's T-bird SuperCoupe with a 66 289 HI-PO implanted into it. it has an auto tranny, headers, and a 4brl. carb. (for dirt cheap. lol)

My question is, how good/bad is the gas mileage going to be, (rough estimate?) if it will get 15mpg or better, that will determine weather I get a truck with a 6cyl. or one with a V8. any help would be appreciated, thanks.