I made up that number to be honest with a little of my own logic. The implement weighs over 600 pounds. I figured hanging 300# off the front would shift enough of the weight back to the front axle to make steering easy and the tractor more balanced. I am sure I could get away with less since I'm just moving it around on flat land.
I like your braking idea. I wish I had thought of that when I was trying to move this thing from the original owner's house! That would have helped a lot actually.
As for the overloaded tractor, if I'm thinking about my physics lessons correctly, the fact that I can pick up the front of the tractor with one hand means that the entire weight of the implement AND the entire weight of the tractor is currently on the rear axle. The front axle is happy as can be right now -- no weight up there. So, putting weight on the front, in my thinking, actually shifts some weight back off of the back axles. I don't know the exact math here but in my thinking, putting 300 pounds of weight on the front moves the weight balance of the whole thing forward and relieves some pressure off the rear axles too... Maybe I'm wrong about that, but it just seems that if you pull the front back down, you're shifting some of the weight of the engine and tractor back to the front axle too, thereby relieving the rear axle. It may not immediately make sense, but adding 300 pounds up front should put more than 300 pounds of weight on the front axle because you're shifting the weight distribution forward. Someone tell me if I'm nuts or if I'm right.