Ford 300's use umbrella seals, as do most older Ford V-8's. Order a set for a 300 Ford. They're cheap. The stem diameter is the same - 11/32. Some Ford engines use a different style on the intakes vs the exhausts. I don't remember if the 300 is like that. No machining is required - just coax it over the end of the valve stem and push down over the guide. When the valve moves it will push it up the stem - this is normal. These give superior oil control over an o-ring in the retainer but not as good as a positive seal pressed on the guide.
