The 61-67 Econoline short bodys tend to be nose heavy becouse the rear bumper is only about 6" from the rear wheels but they have alot of waight in front of the front axle. Ford had to put a waight above the fual tank and it helped keep the rear down an the ground for the most part exept under real hard braking when they are empty.
Between the extra 18" of body with a lighter waight and fuel tank moved back to the rear bumper really helped balance the van. The Supervans only waigh about 200 lbs more then then the short body. Thats with a larger engine (240 ci I-6 1-bbl carb), brakes larger 9" rear axle, heavy duty suspention and longer body. My 66 has the optional front swaybar aswell. It handles very well can carry alot of waight and gets good gets milage (26mpg) at the same time
Ford left the rear axle in the same place since it was cheaper and maintained the same 37' turning radius since they marketed these vans as city delivery vehicles. You just can't tow a heavy trailer with it. It was built for hualing not towing
here are a couple ads from back then
