A possible factory adapter could be found at your airport. Airport? Yes Airplane 'tugs' were buld using flathead Plymouth 6's up to the late 60's. A thick aluminum adapter puts a big block 727 trans on the back. Mine came from the Detroit area, and even with shipping to So Cal it was less than $500.00, just a suggestion.
Best, easiest and cheapest - sell it and buy a car with an automatic. If time and money are no object and you want to keep the original engine, they did use that engine with a Torqueflite automatic 1957 - 59 and with the 2 speed Powerflite 1954 - 56. 73RR has an adapter you can use to install a newer trans, they make them for a lot of old motors.
You might want to do a cost trade off replacing both engine and transmission with newer units. Charlie Stephens
theres no auto for the 216/217. bolt in. if you wanrt a flathead and auto look for a dodge or chrysler. ot do as i did get a stock 318-904 and drive the hell out of her.