Quote (from the notes which accompany the form to obtain a retention certificate):
"When you are ready to put the number onto your vehicle, or that of a nominee named on your V778 Retention Document, you should apply to your local office by completing the V778. (The procedure is fully explained on that document)".
So it appears that you don't require any additional forms; you simply fill in the relevant information on the retention document. (The V317 form assumes that the plate is still on the donor vehicle).
Chris