Chances are, whomever installed the engine had the heads reworked when they did. LP is a "dry" fuel- - - -there's none of the lead anti-knock compounds in it that the gasoline used back then had, and the byproducts of burning tetra-ethyl lead created deposits in the heads that helped prevent valve seat erosion. LP engines have always had to run Stellite seats, at least during the 60-something years I've been building engines.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!