Running too rich-can’t figure it out

You guys must be sick of me. THe rare times I post, it’s the same issue. I can’t seem to fix it. Here’s my problem (again)
1976 Dart 225 all stock 904/2.73 rear

Idling too long or driving in D (3rd gear) typically 40 mph or less my car starts fouling plugs (starting with cylinder 1) and progresses to worsen fouling cylinders 2, 3 and so on.

My fix has been to:
1. Remove spark plugs every 50 miles, clean with brake cleaner, regap, reinstall. Short time fix.
2. Rebuilt the carburetor (H1945) completely. Also changed out plugs, rotor, wires.
3. At this point I pretty much have the idle mixture completely seated, out just a little and it’s helped a lot, but I’m worried of running her to lean. **If I seat the idle mixture screw all the way in, the engine continues to run. It does not shut off**
4. My latest solution is just drive everywhere in 2nd gear probably spinning 3000rpms around 40mph and this has solved all for now. I’m guessing it’s happy just burning all the extra fuel and not fouling plugs, but I do worry at these rpms danger of running too lean.

I know it’s running too rich, but I can’t seem to fix. As stated I rebuilt the carb, new floats, jets, needle /seat. Don’t know what I’m doing wrong.
Any other suggestions?