When you say "a differece", do you mean performance or looks?
If you mean performance, I'd ask about your suspension. Do you have stock .92" (or smaller) torsion bars? Stock rear springs? Aftermarket anti-sway bars? Anything else to increase roll stiffness?
If your suspension is basically stock, I'd say the tires won't become a weak link and cause dramatically worse handling - you possibly won't notice the difference. You didn't say what size 60 series tires you'd be abandoning, but the car might sit slightly higher with 70's, but that's easily fixed.
As for looks, different strokes for different folks - I'll let you decide about that!