[Daves55Sedan]

To answer your question, the 312 engine will bolt up to a Ford-O-Matic that was used with a 272 or 292 V8. these all had the 12" converter. The 215 and 223 I-block 6 cyl used an 11" converter and had a smaller bellhousing.
Actually the air-cooled converter was discontinued in Late '56 for the '57 model changeover. All '57 cars with V8 had the new water-cooled converter. The air-cooled converter was still used with 6-cyl engines all through the 1950's as a water-cooled version was never produced for the 6-cyls at least through '59, maybe later.
Yes, I do still use my front engine steady-rest on my 272 and on my 223 engines. Do you mean people are leaving these off?
dmsfrr is right. You can't trust anything you read on e-bay as far as what they claim parts are for.