My money would be on valve stem seals. Ford uses the usual umbrella style which eventually get hard and break. Also, many, if not most Ford heads, that I've owned or worked on will show valve guide wear. Neither of which will show up in a compression check. A vacuum gauge will only show at best (worst?) a minor needle tick with a single excessively worn guide on one cylinder. The rest will be worn as well, but combining a split stem seal and guide wear will equal oil usage and smoke and time for a set of heads or a full $$$$ rebuild.