JED 1.6.0+ - Regular custom overworld

{
    "dimensions": [
        {
            "dim": 123,
            "dimensiontype": {
                "id": 123,
                "name": "Old overworld",
                "worldprovider": "WorldProviderSurfaceJED",
                "require_exact_match": true
            },
            "worldinfo_onetime": {
                "generatorName": "default",
                "generatorVersion": 1
            }
        }
    ]
}