I would strongly discourage the use of positive valve stem seals at all for a 39-62 Chevrolet engine. They work too well for keeping oil away from the guides.
There apparently is not enough oil moving around under the valve covers on these engines to get past the positive seals to lube the valve guides and they wear out fast.
I learned this the hard way.
The machine shop talked me into them. I should not have listened.

1952 5-window - return to "as built" condition | 1950 3100 with a 235 and a T-5 transmission