HI and welcome. I'm new here myself. I have a 78 bench seat with a 4 speed. Did you check to make sure your alternator is working properly ? Also you could try what I did to get around the cats. just tell the shop its a 77. Most don't check.
Welcome from a fellow Gbody owner from Iowa. What town are you from? Alternator's are known to cause that kind of problem. Unhook the alternator and see if it drains the battery. A nother test you can try is, take the positive battery cable off and lightly tuch it on the battery post. If it jumps a spark pretty good you have a draw. Pull fuses one at a time and check for the spark. There are a lot of us guy's spread out through Iowa if you need a hand.
So my electrical draw seems to have been my old oem starter. I switched to a high torque starter and haven’t had the issue so far this year. The battery has been hooked up for a few weeks and it started right up today, well as good as a carb will that is.