I don't think the new tin is recommended for your type one. That tin is for a type three and used to be called "super tin". Time has shown that it doesn't improve anything on a type one. Definitely use the existing tin. It sounds like you had a sudden change in your engine cooling. This could be because it was hotter. It could mean that your temp gauge or sender are failing too. Drive the car hard on a cold morning and check it then. Remove your gauge and sending unit and suspend your sender in a pot of boiling water to see if your gauge reads 212 f. (you'll need a 12 v battery for a power source) If it checks out pretty close then start looking for other sources of overheating.
Hope this helps...........Bruce