If you have the room under the hood, install a 4-holed spacer under the carb. The thicker the better. I've had a stumbling 650cfm Carter Performance carb on an Offenhauser intake on a 383. Installing a 2" four-hole spacer cured the stumble immediatly. As I didn't have enough room for a normal 3" open aircleaner I went down to a 1" 4-hole spacer. The off-idle stumble came back ever so slightly, but I was able to tune it away.
