Transmission Recommendation for a 318

A 904 will be fine; but since you asked about overdrive, may I suggest a 32RH/A999 lockup instead- it's not overdrive, but it has a lockup convertor which will drop your rpms a smidge. And it's a bolt in with all your existing (you currently have a 904, just the /6 version) shift linkages, speedo hookup, and driveshaft; as opposed to any of the OD Mopar units which will require crossmember cutting/fabrication and floorpan modifications, a custom driveshaft, and whatnot.