I hit 61.7 in a Stampede loaded with a mm7700, 10 cells (6 more under chassis) and a 4oz Garmin GPS seen here.
One suggestion...front end weight. The RPM bumper is a sail at high speed...dont know about upside down, I'd remove it completely and drive it carefully. Also, I had the Garmin strapped to the front, I had an earlier incident where I flipped over backward on a 50 mph wheely.
Added weight and problem solved. Good luck...and bring parts.