I am also worried about tipping. I did something that perhaps was not the brightest idea. I went stright up the steepest slope I have very slow, in 4WD, with the FEL low to the ground and a Box Blade (about 400 lbs) on the 3 point hitch (with my ROPS and seat belt of course). I slowed the tractor down at the most sloped point and got the rear wheel spinning. It was my test to see the point at which I would loose traction. I would LIKE TO THINK I would loose traction on a slope before the front would rotate up and over.
I also have my rear wheel filled, I hope you have done the same. As far as going across a slope, it looks like 45 degrees is about the point of tip. This is based on my glancing and judging where the center to gravity is and the midpoint of my wheels. But you also have to have margin for dips, pholes, etc., By the way, I also have my rear wheels entended the max., about 10 inched beyond normal.
BE CAREFUL!!!