While you are correct that mathematically, front spacers would add something to tractor stability in theory, they may not make a practical difference.
Generally, but not always, the ground does not shift enough over the length of the tractor for the front axle to reach a stop, the point at which the wheel on the low side would contribute to stability.
A tractor is in a dangerous situation when, on a side slope, the high side rear wheel lifts, and the tractor starts to tip in the down hill direction. Once this movement starts, momentum is involved along with position of the center of gravity, which is outside the point of stability. When the front axle stop is hit, you are correct that if the wheel is farther out, there will be more effort required to maintain the tipover motion, than if the wheel were in the standard position. But It is likely that this difference will be enough to stop the tipover in most cases. Could it be? Yes. Would it be? Probably not.
The most important thing is to keep the rear wheels on the ground. Then the spacers on the front become a moot point.
Personally, if I were spending the money, I would put the cost of front spacers towards buying wider rear spacers. That would help keep the rear wheels on the ground, rather than hoping that front spacers would stop a rollover in progress, which they likely would not do.
