There were only 163 440 R/T convertibles built and a few probably no longer exist so when you find one, you may not get much say in options and prices may not be very flexible. On the other hand, there were 692 of the same cars in 383 form giving you more choices, better pricing, and likely a shorter search.
You probably already know this and I certainly don't want to rain on your parade. I hope you find exactly what you want soon. You have great taste in cars!