If you pull the shifter boots (outer and inner), you may be able to fill out through there without actually pulling the shifter out of the way. Since you have no way to check the level, you'll have to drain the trans until empty before installing the drain plug, then pour the capacity amount into the trans as specified by the service manual. Make sure you use GL-4 gear oil. If you use GL-5, make sure it specifically states that it is "safe for yellow metals."