I put some easy miles on the car yesterday; I took it on the interstate for the first time. No full throttle application, I just did some cruising.

I have completely eliminated all exhaust leaks I had at the joints. I have ensured that all the nuts and bolts relating to the intake manifold and header are tight and there are not leaks.

While I was driving I took a couple more AFR recordings. With the SM needles, I am looking pretty darn good at idle - very near 14.7 to 1:

My carburetor linkage is hanging a bit. When I drive normally, and stop for a traffic light, the idle is staying at about 1000. If I rapidly tap the gas pedal, the throttle plates drop a touch, and idle is steady at about 850. I have verified that both carb linkages are hanging... by what looks like something less than .010". When the linkage is hung and the car is idling at 1000, I can put my finger on the carburetor shaft linkage (each one) and force it to shut fully (move the linkage the last few thousandths to contact the idle screws). I think this is likely to go away when I get some more miles on the car.

Anyway, during cruise, I am definitely lean with the SM needles - around 16.2:

I don't notice any surging when this is happening. All seems well. However, if I depress the gas pedal anywhere from slightly to moderately, the AFR goes a bit more lean and I feel that hesitative state.

I still plan to wait until I am at 500 miles before I attempt to do anything else. I am only 50 miles away from that point. Then I will do a leak down check, an oil change, and check valve lash. If the engine is fully broken in at that point, I will need to figure out what to do with either SU spring tuning, or needle tuning.

Disconcerting that your AFM numbers went a little leaner when you tightened everything up. The tightening might be related, or it might be a red herring. But disconcerting either way.

So did you ever try giving it a little bit of choke enrichment with the lever while you're driving? Does it react as expected and the numbers come down?