I've worked with a couple 1 ton trucks in my day, that had a home made box. What happens is you end up filling the truck to over flowing and then pulling the chipper along with that huge load of chips.
You can do it, but you're pushing the limits, that's with a one ton. If you made a small box, you might be able to use a 3/4 ton, but a 1/2 ton would only be safe pulling the chipper, or a small load of chips, not both.
Maybe you could chip the brush, spot the chipper, dump the load and go back and pick up the chipper, but what will happen is you'll end up just hauling the oversized load. It's very dangerous.
In Wisconsin, the DMV is really clamping down. You have to have all the numbers on the side of the truck, the truck inspected, everything, and they have random stops where they block off the road and inspect and weigh every commercial vehicle. Even guys going fishing get it.
They do it because hauling loads is so dangerous, if not done correctly.
Another issue you may have is getting a 1970 truck up to safe working condition.
You might want to take a minute and go to the fatalities section of this site and read about the chipper that came loose recently, killing almost an entire family.