I have a 354 Hemi out of a boat has a sticker on valve cover says it has sodium filled valves in it
I remember a old time mechanic saying the same thing on the H.D. 354 Hemi truck motors with 1/2 stem valves being sodium filled back then, prior to 1959
Mr.Cab Racing and winning with Mopars since 1964. (Old F--t, Huh)