Without those last few digits, you're not going to be able to add those cards to the website. If you still have your Target receipt, you can check to see if the unique serial number printed on the receipt matches the sequence of numbers on the GCs. If so, those could be the missing digits that accidentally got scraped off.
The GCs are not worthless because the magnetic strip can still be used, so you may just want to save yourself the aggravation of a phone call to the Disney Gift Card Hotline. They will not be able to assist you because you don't have the full GC #s and you didn't purchase them directly from Disney (where they could have traced the GC # based on your receipt). And Target's customer service is a joke.
Good luck!