What kind of head is it?

It is a BBM head, probably 2016 or 2017 vintage.

Why are Fel-Pro VS 13049 R rubber valve cover gaskets 3/8" longer (too long) than Fel-Pro 1632 cork gaskets?

CGI is created by pouring the right mix into the mold and then a proper heat treatment of the block.  It is not possible to only make parts of the block this way unless you design it for wet sleeves.

Blown power valves are talked about like an everyday thing, in 40 years I can't recall ever holding one.  Yes, I have put the back side of plenty of PV to my lips and gave 'em a "kiss", yep that one's OK.  Leaking PV gaskets - yes.  All the friggen time.  Worse with 10% pump gas.  Also, if the carb is anything "new", like last 10~15~20 years, it will have a check valve in the base plate to protect the PV from a backfire. 

If you want to check the power valve, simple test - run the engine, stop.  Carefully remove carb without tripping the pump shooters,wipe the base plate with a rag and set the carb on a coffee can or similar - quart sauce pan, etc.  Let it sit for a while and check the base plate around the vacuum hole that leads to the PV chamber.  Fuel wetness or drips into the pan = leak from something, lots of time the gasket can be a bit off center and not sealing correctly.  This does not always condemn the PV, the metering block to main body gasket can cause all kinds of trouble if not sealing well.

Also, on a normal engine if you continue to open the idle mix screws, you will see a steady drop in the vacuum reading, not a bunch of jumping around.  So a leaking PV will cause a low vacuum reading, relative to whatever is the normal level.  Same with lean until you get to some point where its too lean to idle well. 

Quick vibration of the vacuum needle screams valve seats, guide issues,  etc.  Yes, the carb can't "think" with all that going on.  If it smooths out at increased RPM, for sure it's in the valve train.  Maybe a cam lobe, that might be a stretch.  As an example, on ly dual 600 t-rammed 357C with the good size hydro roller, 1200 RPM idle, 37 degrees of timing runs right at 12 in/hg steady when everything is close to right on the idle mix, plate positions and such.   

Here's another good list of suspects I ran across on the web:
WHAT TO LOOK FOR,
(paraphrased from a rebuild manual)
1. A steady reading between 16 and 22 in.Hg. at idle...
This is normal for most vehicles.
Radically cammed engines have lower, less steady readings.

2. Normal range at idle, with sporadic drops below normal,
This could indicate a sticking valve.

3. Normal range at idle with needle vibration of about 2 in.Hg.,
This could indicate an ignition problem. Check plug gap, dwell (yes, electronic ignitions have dwell too), cap, rotor, and plug wires.

4. A steady reading slightly higher than normal.
This can be caused by a dirty air filter, or overly advanced ignition timing.

5. A steady reading 3 to 12 in.Hg. lower than normal.
This could indicate one or more of the following conditions,
Intake or carburetor vacuum leak, Late ignition or cam timing, Worn piston rings.

6. Gauge needle drifts slowly over a range of 4 to 5 in.Hg. at idle.
This could indicate an idle mixture that is too rich or too lean.

7. Gauge needle fluctuates rapidly between 10 and 21 in.Hg. at idle.
This could occur when one or more valve springs are weak or broken.

8. Gauge reads normal at idle, but drops slowly as engine speed is increased to 2,500 RPM.
This could indicate a restricted exhaust system.

9. Gauge reads below normal and fluctuates rapidly over a range of about 3 in.Hg. at idle, then the needle becomes steady as engine speed is increased.
Worn intake guides usually cause this reading.

#7 Is the closest to my condition.  I can pull the valve covers and look.  I know what a broken valve spring looks like, but how do I find a weak valve spring?

The thing is, if there is something fundamentally flawed with the engine in the ignition system or the valve train that is causing it to run rich (or misfire) with #72 jets, then #74 jets are not going to fix it right?  Ignoring the O2 data, given the symptoms, where would you start to try and get this thing running right?  Would it be jetting or something else?

The engine didn't "feel" right, like it was "unhappy" before I installed the wideband O2.

It has a lot of vacuum oscillation at idle, like 5 inches peak to trough, which is a lot considering that the peak is about 13 inches and the trough is like 8.  The needle bounces all over the place and so far, setting initial timing doesn't seem to help.  While the car ran OK without backfires or stalling, fuel economy was less than I thought it should be, it certainly smelled very rich, and throttle response seemed lethargic.  At the time I was just running an out-of-the-box Holley 4160, so I thought there was room for improvement.

Perhaps a combination of this cam and a single plane intake manifold is just always going to run like this.  I hope not.

On the dyno, with a different carb and a different distributor AFRs ranged from 12.2 - 13.6 during the full pull.  I'm not saying this is the magic range I'm trying to achieve, but it seems like it is at least possible to control the fuel curve on this engine.  Also, I'm not trying to wring out every last bit of power from this engine, I think it was actually running pretty bad and there were significant improvements possible.

So, this year I decided to try and tune it.  I installed the AFR and just drove it around for a while and see what was going on.  It was consistently rich, running from 10-12.5.  On the highway at 2200 RPM cruising at steady speed on level ground AFR was about 10.1 according to the gauge.   From what I understand about the process, after setting the idle mixture for peak vacuum, the next step is to tune the primaries by running at steady cruise just under the RPM where the secondaries start kicking in.  The first sign of trouble was it took a lot of jetting to get steady state cruise in the window, from #72 down to #67.  Then instead of predictable AFRs, they are erratic; swinging between 12 and 16 with very small changes in throttle and RPM.  So I've gone from sub-optimal to very unhappy.  I did not expect to encounter this kind of trouble.  I haven't even started trying to fiddle with the power valve or the secondaries.

Right now I am following Joe's advice.  I've cracked open the secondary throttle set point and closed down the primaries below the transition slots.  Right now it won't stay running without me on the throttle.  It might idle at 650, but it will definitely not idle at 500.  I am going to pull the carb back off the engine and open the secondaries some more.  Is there any fuel coming through the secondaries at idle?  I do not have idle screws in my secondary metering block.

Yes, maybe a reduction in mechanical advance curve would help.  Right now I'm still trying to tune drivability between 1000-2000 RPM, so total advance isn't a factor yet.  Also distributors are as much a mystery to me as carbs.  I wish I was just changing numbers on a table, but this is what I'm working with.

If I knew how to tune the carb to make the engine run right I wouldn't need the oxygen sensor.