High RPMS at 55 MPH

Im wondering why Dads 67 cuda is revving so high...Only going 55 MPH and we are at 4k in 3rd gear... specs are 273 4 bbl, 3:23 gears and above stock torq conv. No idea what the stall is on it but the converter is new. Also havent verified the rear gears, but Dad is 3rd owner and its fairly original so I am assuming orig 3:23 gears

Here is the build/resto thread if that helps
http://www.forabodiesonly.com/mopar/showthread.php?t=178255

Thanks
4spdragtop n pops