Well, I'm kinda biased as I have a reverse rotation tiller. If it was me I'd probably till it up good and put down some weed killer like roundup or some such to take care of anything that wasn't turned under by the tiller. After that had a chance to degrade so it wouldn't kill the seedlings I could put them in with some confidence.
Turned under the grass will take awhile to grow back up, probably enough to give the seedlings a good head start.