Yes, my ballast box is 707 Lbs, but it's quite close to the tractor, as I built it primarily to also be the trailer hitch for my water wagon, while carrying a tote of water on the forks up front ... It acts similar to the backhoe as far as balance.
600 should be OK, my Backhoe weighs 651 Lbs, but again hooks closer to the tractor, but also sticks out behind aways ...
If you do a ballast box, AND you don't have a quick hitch, you will quickly find that a furniture dolly is very helpful to set it on, if you have a hard surface, wrestling a heavy attachment like that to line up with the three point is something you will probably only do once!
The farther out it hangs, the less weight you need (remember the teeter totter?) but also is easier to bump into things turning or backing up ...
My brush hog only weighs 518 Lbs, but is 8' long, and it is much heavier than the backhoe as far as how light the front end reacts to steering, I need to put stuff in the bucket to mow with it!