Yes, they make a difference. Consider adding front torque boxes (both sides, even if not a 'vert), monte carlo bar and export brace. These cars weren't known for being especially stiff, body-wise, when new. Its now 45+ years later. It goes without saying that any rusty floorpans should be replaced/repaired first. A bigger front bar and the Shelby drop are also a very big plus for handling. I also added Traction Masters bars like the Shelby's had, some go with Cal-Tracs, if they go this route. Roller perches won't 'stiffen' the car but help to make it 'smoother'.
Even wihout additonal HP, I've seen the cracks at the quarter windows. Anything you can do to stop the flex will be something you will feel as it will keep the tires in place.