Have same issue on a 78. I believe the shaft is worn but I haven`t taken the carb off yet to investigate Already replaced the choke and control. I think the butterflies hang up
I bought them from a guy selling them for $20 plus shipping on a facebook site. No idea on the cost. They matched up well to my originals but I never needed them. 3788273 is the mopar part number and very are hard to find so I bought them as spares.
This company on ebay sells these for a good price. May want to check and see if they have what you want. 1966-72 Mopar B & E Body Black Aluminum 26" Radiator Roadrunner Challenger Cuda | eBay