Looking great 
I have ordered Animal Crossing from Japan in hope that it will have the MN63 chip. I already written a test version of the MN63 support but since I can't test it I don't know if it works or not.
You'll find saturnus ED64 Saveswap tool inside the ed64_archive_v3.zip here
And remember you should not plug the USB cable directly into the Arduino when the N64 Adapter PCB is also plugged into the Arduino. So to upload a new version of the Cart Reader code you must first remove the N64 Adapter PCB. Because otherwise the N64 cartridge will get 5V instead of 3.3V. Although it probably won't do any damage.