Nothing is simple there either. Chev offered a SM318 with a Warner OD, but that requires an open drive shaft which means getting a 55 or newer third member. And of course you will have to have a relay to operate the OD. Chev overdrives appear on eBay occasionally. I prefer overdrive transmissions and have been using them for 45 years. You can lock the overdrive out and have the pep around town of direct, but then when you get on the highway you can let the overdrive kick in and get a 30% reduction in RPM.