Big Block in a K5

[Big J]

I've got a 72K5 and am thinking of dropping in a big block. I have access to a 454 punched out to a 461. Its a new professionally rebuilt engine with all high end internals. My uncle has it and said I can have it for 1800.00. Is this a good deal? My next question is what kind of mileage will this thing get. Its a factory 4 speed, I think it has 3.07-3.73's in the pumpkins and I'm going to run 35's or 37's. My thinking is the big block will be a torque monster and make up for the gearing. Maybe it'll hit around the 17-18 mpg mark. All opinions will be greatly appreciated.
Thanks for the help,
Big J