The plug leads being swapped are a worry. This is when I'd be checking everything - plug gaps, timing, throttle stop on the carb, how much adjustment you have on the distributor etc. It is easy for a novice to start 'adjusting' things just because it seems like it's affecting how the engine is running and then you are miles away from where you need to be. If the distributor has no advance in one direction or another it may have been installed out a tooth, or worse - 180 degrees out (it's closer to 170ish degrees as the distributor drive gear has an odd number of teeth) Check if you can accumulate pressure at the exhaust with your hand (not a super accurate way especially if you know the system is shot and need replacing). A blocked cat will stall out an engine so you are not a potato. Also check things like accelerator cable slack/adjustment. I've seen them with HUGE amounts of slack and winding the adjuster out can instantly gain lost power. This will be something minor you are looking for. You'll get it eventually