Thats exactly what my 7 did when it had the same set up. It was caused by those 2 fuel metering rods that you can see with the air cleaner off and looking down into the carb. I first switched to a weber [we don;t have have smog inspections here yet], and found it to be ok but a little picky to set up especially for cold starting. It goes down to minus 35 here in the winter. Then I switched to a Holley truck avenger 470cfm. It worked real well but was a little big for a 258. After that I put a 304 in the 7 and put the Holley on that motor where it remains to this day.
Another card swap you could do is a motorcraft 2100. Just do a search thru old posts to find the info. I don;t know where you live but if you have emissions testing you are prob best to keep it stock. My friend has a 77 cj7 with a 2 barrel BBD I gave him and he just uses a can of compressed air and using the thin plastic tube in the nozzle directs the air into those 2 metering rods to keep them clean and that works for him. They are responsible for the idle, once you step on the gas a different circuit in the carb comes into play.