Surprising that you are having high RPM's on the Fury. I had a 1965 Fury III convertible and that had 2.76 in the back and it cruised at 80. Check the rear end to see what the gear ratio is. There might be a tag on the rear end attached to one of the bolts on the chunk. If not you can do a check by lifting the back end of the car off the ground. Put a vertical mark on the bottom of the tire and one on the yoke and pumpkin. Rotate the tire (in neutral after the car is secured) until the mark on the pinion/pumpkin line back up again. Look to see where the mark on the tire is and that should tell you your ratio (pretty close) I know my explanation is a bad one, and someone will be able to correct it or add to it. It is one of those things that I can do but explaining it is different. The 2.76 gear is the highway one that went into a lot of big Chryslers and they are pretty easy to find. If you have a sure grip then no doubt you have a lower gear ratio. You VIN will not tell you the gear ratio, but your fender tag might....Maybe. The line sheet from the car would tell you the code for your rear end if the car has one. They are typically under one of the front seats, or possibly the back seat, sometimes on top of the glove box liner.