Hints on improving rear end

[mhjo]

I have just arrived home from holidays, driving the Mustang through Germany on autobahns. We where two passengers w luggage in the trunk.
I discovered that the car was unstable in the rear at 70 mph and higher.

I'm taking the car for long trips next year, and I wonder what improvements I can make on it before I go.

At the moment the rear is "untouched", w original leaf springs and no bars.

Any good hints on impovements??

 

[NorCal66]

New suspension will make the biggest difference. Subframe connectors will also make the car stiffer and more stable.