You'll have to size the case just enough that the action will close completely and lock up the same each time, the case can protrude from the chamber slightly and it can still lock up normally, it just depends on the barrel to frame gap. After firing a round, close the action with the spent case in the chamber, if it closes with no visible gas when looking across the barrel to frame gap, then you can neck size only, it should look the same as with an empty chamber, but if there's any gap at the top of the barrel, you'll have to slowly adjust the sizing die into the press a little at a time until that gap disappears. Or you can adjust the die in until the brass is flush with the chamber mouth which should give you acceptable headspace, the SAAMI max tolerance is .006" which would be a very noticable gap tween barrel and frame.
Tim