Many years ago I had a '67 Pontiac Catalina with a High Cpmpression 400 2 Bbl., TH400 and 2.29 gears. It weighed right about 4,200 Lbs. dripping wet w/o driver. It would lay down a black stripe for 2 blocks, bury the speedometer in a little over a half mile and got an homest 20 MPG. Now time may have "fuzzied" the details a bit
but if it has they aren't far from reality. You'll be fine.