Figured out yesterday that the body value needs to be JSON encoded, not passed as the underlying object. The correct form is like the following (value has been abbreviated).
{
"spaceId": 9867790169,
"status": "draft",
"title": "Equipment Sales",
"body": {
"representation": "atlas_doc_format",
"value": "{\"type\": \"doc\", \"content\": [{\"type\": \"paragraph\", \"content\": [{\"type\": \"text\", \"text\": \"foo 3\"}]}]}"
}
}