Filled tires at lower levels

[vtsnowedin]

It is not obvious to me that the axles are at the CG of your tractor. I think 75% fill puts all of the liquid below the CG.

Think about wheel spacers to widen the stance of the tractor. This provides tip-over resistance without adding very much weight. If you want weight for traction this doesn't help much, but you seem to be more interested in stability.

Not quite. Usually the COG is above and in front of the rear axles but varies of course with ballest placement and loader position. Here is the best referance I've found.
http://www.nstmop.psu.edu/tasksheets/4.12 Tractor Stability.pdf
I also like spacers and 75% rimguard fill. They work great.