It sounds to me like you may not have enough stem to guide clearance. The older engines required more clearance than modern engine because fatter stems expand more than modern stems. When I do valves, I mike the stem and then use a ball type small hole gauge for initial check of the guide. I then use a TESA holtest with a setting ring for a final measurement after I ream the guides. I give them the maximum clearance from the spec sheet which is kind of contrary as to what is done these days. But I don't have stems to seize. When stems seize, push rods bend, and even worse, valves can hit pistons.
