I'll try to answer this even though I'm no expert on this subject. As I understand it, the 7.50x14 may be replaced with either the P205/75R14 or the P215/70R14. You will find both radial equivalents have a smaller rolling radius than the 7.50x14 bias ply. I have never seen this published in any of the tire vendors literature. They do publish the outside diameter of their various tire sizes, but that doesn't mean much when you load them on the ground.
What this means is your radial equipped car will have slightly less ground clearance than it had with bias plys. Not much, but measureable. Here is another change you can expect with radials. Your engine speed will be higher to maintain the same true road speed. An automatic trans car with bias ply tires will register 2500-3000 RPM when cruising in the 65-70 MPH range. The same car with radials will be in the 3000-3500 RPM range at the same speed.
I hope your dissatisfaction with the bias ply tires isn't really caused by a worn idler arm bushing, weak shocks, or some other suspension or steering component.
Tom
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