I never noticed the actual ballpark distance on that but I'm thinking it should move more than 1/4". All you posted is correct. I'd loosen the dist clamp & turn the housing so you can see down past the 2 plates to check both slots & see if the pins are not hung up or the slots have not been welded up as you twist the rotor again & you'll have to turn the housing to 2 different places to check both slots and as you know you'll need to retime when your done. If nothing is obvious you may have to take off/out the reluctor/plates again. EDIT I checked my '67 "Glenns" manual & no 340 that early but every dist I saw has way more than 5 deg mechanical (slots) so we're missing something right in front of us