Yep, something still isn't right. Using a RPM calculator, with a 3.70 final drive, a .68 overdrive and an assumed 26" tire height you will be turning 2276 RPMs at 70 mph. To get 3383 RPMs @ 70 mph would require a 5.50 rear gear
... I think not! Possibly your tach is fun-goo'd or your speedo is waaaaay out of calibration.