so you have not actually experienced the rear end skidding or sliding out ?
this is just a worry at this point?
sounds like and original /6 torsion bars and rear leaf springs. With 13” rims, it probably has small 9” drums front and rear. Does it still have a /6 motor?
With those 185/70/13 tires at 50-60mph curves like you see in my video, you’ve got grip and sidewall roll over issues. You need to address that to some degree first.
On curves or freeway on-ramp with post MPH signs are you exceeding 10-20 mph over posted?