You just need to plug the line actually AT the valve itself. Anything before that doesn't really matter. You just don't want the valve opening. The stuff in between is probably just a thermoswitch that lets it operate only when the engine is hot. At least that's how my '84 model is. The plumbing to the tranny is totally separate, I'm not sure why your shifting was affected. Mine's fine. Make sure you didn't accidentally dislodge a different line while poking around in there...
