How lean is too lean?

7milesout said
The way mine runs even at these high / lean AFR numbers, it runs very strong. If I didn't have an AFR gauge on it, I would not even know it was lean, as it runs great.

Bingo. That's using your noodle. I know, same thing I told Mopar R&D, but its true. Tune to performance.
see this post: How to properly drill carb jets?

7milesout said
my 14.0 - 14.5 AFR at cruise immediately gets leaner when I open the throttle. I'll hit 15.5 - 16.0 AFR trying to pick up speed. So here's the question: At this non-WOT condition, where is the threshold that XX.X AFR (lean) is OK, but YY.Y AFR is too lean and could lead to problems, even at this power mode / cruise rpm? Or … is this lean condition not an issue because I'm not at WOT?

Leaner with more throttle: Exactly and then some. Its what is supposed to happen.
We have to go to the books based on testing and experimentation for the automotive and aircraft industries.
This from Larew's book on carburation:

Notice the fuel air mix gets leaner as the load increases to about 60 % max load. Nothing magic about 60%. An engine with more efficient part throttle could be leanest at 80% max load and run better.
More fully explained here:Need Opinions. Have nothing to compare to.

Threshold: You'll have to experiment to discover how lean your particular combo will accelerate better for a given load. Generally if you have lean high speed cruise dialed in, the part throttle acceleration will be darn close. So, tune for steady state situations first (idle, high speed cruise, low speed cruise, WOT) then work on transient situations as needed.

To get the high speed cruise, run steady at 60 - 65 mph with leaner jet - rod combos until it revs and tries to die. Find a safe place to do that! When it does - you must slow down. Don't punch it or anything stupid. Might even have to pull off and restart. Drive back to the shop at a slower speed and richen the jet-rod combo for part throttle and while keeping the WOT unchanged from the best at the dragstrip. The Eddy manual has a chart to help with that.

7milesout said
I have my Edelbrock 1405 equipped LA360 cruising at say 45 - 60 mph in the 14.0 - 14.5 AFR range. But when I go to accelerate (not WOT, just accelerate at all), what happens when my foot pushes on the pedal? I'm not (directly) adding gas, but I am directly adding air by opening the throttle valve. So, my 14.0 - 14.5 AFR at cruise immediately gets leaner when I open the throttle.

Actually at the speeds your describing, the increased pull on the fuel is immediate with the increase in air flow. At interstate speeds change in air velocity through boosters causes an immediate change in the pressure in the main fuel passages. The air bleeds are used reduce the pull on the fuel. Its a well matched system giving an engine exactly what it asks for immediately.