I've cut the rear fender on about 6 Connies. (12 year time frame)
I always cut it just the fender just below the level of the reflectors.
I've used various methods to re-attach the plate. Bobbing without raising the license plate seems wrong to me.
Most recent/simplest plate mount is; Remove the license plate light. Install the license plate on the stock license plate bracket,,, using the lower 2 holes of the license plate.
Works fine.
NOTE: I have never been hassled about the lack of a license plate light. If I were, I'd do as MOB did, or buy lights that mount in the License plate holes.
Ride safe, Ted