I have a 68 Coronet R/T that I have magnum wheeels with P225/60/R15 on the frt. and P255/60R15 on the rear. (which I like)I want to put red lines on it but I can only find them in 70 series
P 215/70R/15 and P235/70/R15. I need some input or
on how much difference this will make,
good or bad or should I go back to 60 series? Any imput will be welcomed!

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!

Both in looks and performance. The suspension is completely stock R/T suspension. I know the 70 series is a taller tire but don't know what other differences there are. And or will it handle the same as with the 60 series tires? Which perform fine.

Compared to a 60 series, I can feel a 70 series side wall fold over sooner and they have an overall less precise reaction to steering input - you steer and then have to wait for the tire to catch up and take a set. I wouldn't put anything taller than a 60's sidewall on any car that is going to be driven in a sporting manor.

Diamondback will put a redline on any tire made

Sorry, I don't have there contact info handy