A bad indication on a leak down test on an engine with good compression is more likely to be a valve problem then a ring problem. If you are changing the heads it doesn't matter. There are tons of Mustangs with that many miles on them with HCI combos. That doesn't mean your engine is necessarily the optimum candidate, but it's likely fine and it will work fine. I've seen high mileage engines perform better on a compression test than a new one. If you have been using synthetic oil then it might not have much wear at all.
Kurt