If you install the 3:55 gears you will be able to do what you want to do. That being said, like others have stated, you will need to go through the whole driveline to update parts.. Traveling at 65-70 mph you want to make sure your kingpins, tie rod ends, draglink, all bearings, etc. are in GREAT working order.
I have an SM420 with the 3:55 setup and can run 70 down the interstate without issue. I still have original STYLE brakes, steering etc., but all are new, and well maintained.
