Nice ditty! You might drop the volume on the mellotron/string parts in the chorus when they come in on the left side, they swamp the vocals a bit.
You aim for the early 60's and so I guess all that reverb fits the bill. As you get it more perfecterest, you could smooth the ending so that all three elements end at the same time, and fade away smoooooth.
It's a very pleasant song for sure. Have you thought about some doo wop or equivalent bgv's?
cheers,
-Tom