I think it can be easily converted from these GTA IV models:
https://www.gtainside.com/en/gta4/cars/145267-2003-cadillac-sixteen-v1-3 (it's the better one imho)
https://www.gtaall.com/gta-4/cars/127933-cadillac-sixteen-v12.html
https://www.gtaall.com/gta-4/cars/127937-cadillac-sixteen-v12-pj4.html
https://gamemodels.ru/files/file/7677-cadillac-sixteen-concept-03 (3D model only)