On a Ford in the late 60's and 70's always look at the serial number or engine tag.
The 1969 Ford Mustang should have the 250 cu in . But I worked at Ford in the years this car was produced. And the 200 cu in was available in 1970 along with the 250 cu in. But, and there is always a But, in production Dates of some Ford engines. These engines are so similar that, depending what engine they had at the factory on the specific day, the car was made; that is what the car got. I remember seeing the 302 V8 with 289 cylinder heads, 289 was cast into the cylinder heads. You may have a late production 1969, that got a 200 cu in. Not a big deal at all, those engines all worked pretty much the same.
The 170/200/250 engine is a great engine. It is so simple to work on. We did a fair amount of valve jobs on these back in the day; and always check the valve guide clearance, as the guides had a tendency to get enlarged or just worn out. We were starting to see the effects of less lead in gasoline back then. Later engines had harder valves and better valve guides. Head gasket surface and cylinder block deck surface should be checked also, any time the head is taken off. The cast iron heads and block are very durable but a machine shop can true them up, with a moderate cost.
You can do the job at home in the driveway; but spend a few $$ and take the head to an automotive machine shop. Why skimp and not be sure if the guides are good or the deck surface won't hold a good head gasket properly. And don't even try this job without a torque wrench.
You have a good engine there, treat the repair seriously and that engine will out live you.
intimeold