Again, before doing anything else or spending another dime, ensure the engine is in good condition. Maybe do a compression or leak down check, verify 100% there are no exhaust or vacuum leaks, make sure the fuel level is correct, all your bolts are tight, etc. If you haven't done any of this basic stuff first, stop and verify it all before doing anything else. As the saying goes, garbage in, garbage out.
My suggestion would actually be to forget about the 02 right now. Normally it functions as a feedback device in EFI programming but with a carburetor and a stand alone AFR gauge it's a one-trick pony and does not factor into the decision to add or subtract fuel - it can't. What are you seeing on the gauge anyway? Since you're looking at plugs it sounds like you're not even relying on the gauge much to begin with. If that's true then it makes no sense to add another one.
Do yourself a favor and install a vacuum gauge to a manifold vacuum source. Drive around with it to see what kind of readings you are getting under different throttle conditions. With a carb, a vacuum gauge will tell you more than an 02. Unless you have a big cam, vacuum at idle should be 17"-22" and should be similar at cruise. If it's way off then most likely the timing is incorrect.
When tuning, don't do anything to the carb before verifying the timing is correct. Do you have vacuum advance hooked up? Need to test that in cruise mode. Shoot for as much advance as possible without detonation then back it off a degree or two to be safe.
If you've gone through all that and verified the engine is mechanically sound and the timing is correct then you might want to go back to a baseline setting with the carb and start from scratch. Probably not what you want to hear but it's easy to get turned around. Edelbrock carbs rely on step up springs to time the fuel delivery and metering rods to alter the volume of it.
