depending on the condition of the motor overall and how it's tuned (and how close to stock the motor is) I'd guess you'll get about 10mpg in town, 14-15 on the highway.
However, since it's not the original engine it may well not be a stock engine (ie, when the original owner swapped in the 455 he may have had it "built" for more power) it may be a real gas pig. Hard to say.
As asked above, does the original engine come with the car because Pontiac discontinued the 455 (in any car) after 1976, so it can't be the original engine (or the owner is confused and really has a 400 but has thought for 31 years it's a 455)? Worth asking probably if you're serious about the car.