Here is a drawing from my manual.
I would be nervous about how much weight I had on the 3pt using this method.
... I would add r.e. your question about wheel weights vs. rear ballast, if your concern is stability while mowing, then wheel weights will do as good or better a job. Wheel weights are further out to the side of your center-line, thereby effectively widening your stance by moving your weight distribution outwards, and wheel weights may also be lower than a ballast box on the three-point, lowering your center of gravity. Liquid-filled wheels have the lowest CG of all, since they are resting on the ground. Presumably you aren't going to be using your FEL to lift heavy loads while mowing, so the fact that wheel-based ballast doesn't help the FEL's capacity may be moot. You could leave your wheel weights on all the time, or load your tires, and then simply drop off the ballast box whenever you were going to mow. I find that the tractor handles a little more roughly with the loader on and no ballast on the back, but it's not really a safety issue, as long as there is some ballast on the tractor to ensure adequate traction e.g. when going downhill forwards.
You could just add a couple of pneumatic wheels on casters to the bottom of your weight box. A couple of hundred pounds wouldn't be a problem depending on the size wheels you get. The box will just ride along behind you with the added benefit of keeping the weight as low as possible. Just not sure that would help with "counter weight" if it's essentially resting on the ground.
HP
...
5) How does a 3 point mounted salt/seed spreader mount........does the height go up and down, or are they mounted in a static way? That is something I had considered, for seeding the yard in summer, and sanding/salting the driveway in winter (in which case, I'll sell the weight box).
...so, to clarify, since my previous post sounds like I was rambling....LOL:
1) I don't really want to remove the loader every time I mow, and even without a FEL, with the hills I have, some ballast might be smart (I am mostly concerned with rolling the tractor while mowing on hills, the tractor is VERY tippy even with an empty FEL on hills.