With that milage, I'd do a compression & leak down test..if those have solid results then go for it as new H/C/I/ are gonna put some umph to it. If the motor was cared for and not beat to death all should be fine. I might drop the pan and replace the oil pump.
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.
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.
You are more checking for even compression. Excellent compression is all the cylinders with 10% of each other. As long as one or two cylinders isn't way off, you are good.