Niles,
Based on your description I would get a 3.5 power valve (not the high flow style). However, before I removed the carb from the car I would verify that the idle screws do make the engine stumble (slow down) when you turn them in (clockwise). I would also spray some carb cleaner in the idle bleed air holes with the engine running and see if they are plugged (the engine will stumble when the carb cleaner interrupts the air flow). If it does not there is something blocking the air bleed holes and you need to find out what it is when the carb is off.
If you do not have a power valve blow out preventer now is the time to install one. Also, check to see that the rear throttle plates are closed at idle, if the rear plates are open even a small amount (more than .010) that airflow can be a problem.
You need to check the acc pump clearance at idle (bolt head to lever at least .010 but not more than .020).
Your vacuum at idle seems low even for a big cam. Have you checked for a vacuum leak? Are you running a PCV valve or is it the correct one? (it may be too high a flow) Is there a lot of fuel in your
oil? ( give the dip stick a sniff, if you have been running real rich the
oil may have enough gas in it to be a problem)
I would not put to much faith in the O2 sensor for now, you need to get it running right so it does not foul the plugs and foul the O2 sensor before you can trust those readings. The sensor must be clean and up to temp. (HOT) before it is accurate.
Hope this helps, Keith