Hi Chris, don't know if that shortening is significant, but I'd change them anyway. While you've the head in bits. it's easy. If you re-assemble and there's a misfire or suchlike, you'll be wondering about those springs and kicking yourself because you didn't change them. If they are original, after 50 years of heat cycles and millions of compressions, they are bound to be a bit weary. Change 'em.