Early 50's Powerglides just used a rod with a lazy Z bend in it direct from the column lever to the trans. No belcranks or nutt'n. Obviously you'd only use one lever on a conversion. or use a 50's automatic shifter lower part with the safety switch on the column? You didn't say what trans...
I'm assuming you want to use the stock 3-speed shifter mechanism with a modern automatic. did this on my 49 chevy, just made a rod that went from one of the stock 3 speed levers in the junction block to the trans, had to bend it a little to work. I'm doing the same thing with my new 49 project except I'm using a cable rather than a rod, and I am eliminating the original 3 speed junction block. I'm just going to put a bearing at the end of the shifter and make up some sort of a mount, then make an arm for the cable to attatch to. no one makes a kit for this.