17" Wheels on a 69 Dart with Stock Suspension

The thing I'd be worried most about is whether or not the 7.25 can take a larger moment of inertia coming from larger wheels and tires. You may want to consider bumping up to an 8.25 or 8.75 rear unless you baby that 7.25 as much as possible.
As long as he keeps the tire diameter the same, at around 26", and the tire/wheel weighs about the same, the above statement regarding "larger moment of inertia" does not apply. And based on all the coversation here, it sounds like he's considering a tire around 225/50R17 and is looking at some nice aluminum wheels. I bet the 17" wheel/tire package is no heavier than whatever came stock on this car. Is a 7.25" rear end the best thing in the world? Nope, but we don't even know what engine this car has, or its intended use. So it may be just fine.