Hi and Welcome . I've also got a 50 Coronet with Fluid Drive. In addition to Andy Bernbaum, try Roberts Motor Parts, 978-363-5407, www.robertsmotorparts.com
For Roberts, and likely for Bernbaum, what's on their website isn't everything they have. Always good to call or email. They'll make sure that you're ordering the right part.
I haven't had any trouble with my trans . . . yet. Good luck. Love that sun visor on yours. I'm visor-free, but not by choice. I like your red wheels, too. In fact, I signed on today to find a source for that classic Dodge red paint. I need to repaint my hubcaps and also want to go from black to red on my wheels.
Again, welcome!