You mentioned you're doing this as an employee. And that your boss had no advise.
Many have told you what you are doing is dangerous, and now that I know you've got 400 bushels behind you, I'm going to tell you it's dangerous.
The owner of the business (your boss ?) is the one who should be experimenting with gears, and brakes. As an employee, you don't paid enough to be the guinea pig. If he's not willing to figure it out (too dangerous), then let him give you a trailer with electric brakes.
As a younger man, I always showed the employee I could do, and did do, the jobs that were dangerous. Doing the job and explaining how to do it with the least exposure to injury. Now I'm 80. I won't do some of the stuff I used to do. But I tell the employee if their concerned with the task at hand, no worries - we'll hire it done.
I don't like the disrespect for your life and limb, as displayed by your boss.