From the monetary side, I say don't swap the tranny to manual. But the fun side says a convertible should be a stick car. Just my opinion from being a convertible owner for 42 years. Mine is a '70 in the same color. It was the first running car I purchased at the age of 15. I still have it. Mine went way off track from being a daily driver. High compression, manual valve body in the auto, high stall TC, too much gear. I say do a junkyard engine and drive it until you're tired of the V6 and make the foolish mistake of doing a coyote swap. And then sell it before you actually start that project. Should sell for a nice profit.
Wow 42 years. That's a long time. This is going to be my first convertible. I think I'll take your suggestion and leave the transmission and just change the engine. I plan to use it as a daily driver.